[feature] Add Hashtag support

[tsmethurst]

Many implementations (like Mastodon) use the Hashtag type for hashtags, but we don't support this yet.

A Note with a Mastodon style hashtag in it looks like this:

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    {
      "ostatus": "http://ostatus.org#",
      "atomUri": "ostatus:atomUri",
      "inReplyToAtomUri": "ostatus:inReplyToAtomUri",
      "conversation": "ostatus:conversation",
      "sensitive": "as:sensitive",
      "toot": "http://joinmastodon.org/ns#",
      "votersCount": "toot:votersCount",
      "Hashtag": "as:Hashtag"
    }
  ],
  "id": "https://mastodon.online/users/FediFollows/statuses/108556046579308599",
  "type": "Note",
  "summary": null,
  "inReplyTo": null,
  "published": "2022-06-28T16:30:16Z",
  "url": "https://mastodon.online/@FediFollows/108556046579308599",
  "attributedTo": "https://mastodon.online/users/FediFollows",
  "to": [
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "cc": [
    "https://mastodon.online/users/FediFollows/followers",
    "https://gts.superseriousbusiness.org/users/gotosocial"
  ],
  "sensitive": false,
  "atomUri": "https://mastodon.online/users/FediFollows/statuses/108556046579308599",
  "inReplyToAtomUri": null,
  "conversation": "tag:mastodon.online,2022-06-28:objectId=63614708:objectType=Conversation",
  "content": "<p>GoToSocial is a new free open Fediverse platform which runs on cheap lightweight hardware and emphasises user safety and privacy. You can follow at:</p><p>➡️ <span class=\"h-card\"><a href=\"https://gts.superseriousbusiness.org/@gotosocial\" class=\"u-url mention\">@<span>gotosocial</span></a></span> </p><p>It supports both blocklists and allowlists, block/allowlist imports, non-federated instances etc</p><p>It&#39;s still in alpha testing, you can find out more info at <a href=\"https://docs.gotosocial.org\" target=\"_blank\" rel=\"nofollow noopener noreferrer\"><span class=\"invisible\">https://</span><span class=\"\">docs.gotosocial.org</span><span class=\"invisible\"></span></a></p><p>The project&#39;s souce is at <a href=\"https://github.com/superseriousbusiness/gotosocial\" target=\"_blank\" rel=\"nofollow noopener noreferrer\"><span class=\"invisible\">https://</span><span class=\"ellipsis\">github.com/superseriousbusines</span><span class=\"invisible\">s/gotosocial</span></a></p><p><a href=\"https://mastodon.online/tags/GoToSocial\" class=\"mention hashtag\" rel=\"tag\">#<span>GoToSocial</span></a> <a href=\"https://mastodon.online/tags/Fediverse\" class=\"mention hashtag\" rel=\"tag\">#<span>Fediverse</span></a> <a href=\"https://mastodon.online/tags/ActivityPub\" class=\"mention hashtag\" rel=\"tag\">#<span>ActivityPub</span></a> <a href=\"https://mastodon.online/tags/FOSS\" class=\"mention hashtag\" rel=\"tag\">#<span>FOSS</span></a> <a href=\"https://mastodon.online/tags/FLOSS\" class=\"mention hashtag\" rel=\"tag\">#<span>FLOSS</span></a> <a href=\"https://mastodon.online/tags/Libre\" class=\"mention hashtag\" rel=\"tag\">#<span>Libre</span></a> <a href=\"https://mastodon.online/tags/FreeSoftware\" class=\"mention hashtag\" rel=\"tag\">#<span>FreeSoftware</span></a> <a href=\"https://mastodon.online/tags/OpenSource\" class=\"mention hashtag\" rel=\"tag\">#<span>OpenSource</span></a> <a href=\"https://mastodon.online/tags/SelfHosting\" class=\"mention hashtag\" rel=\"tag\">#<span>SelfHosting</span></a></p>",
  "contentMap": {
    "en": "<p>GoToSocial is a new free open Fediverse platform which runs on cheap lightweight hardware and emphasises user safety and privacy. You can follow at:</p><p>➡️ <span class=\"h-card\"><a href=\"https://gts.superseriousbusiness.org/@gotosocial\" class=\"u-url mention\">@<span>gotosocial</span></a></span> </p><p>It supports both blocklists and allowlists, block/allowlist imports, non-federated instances etc</p><p>It&#39;s still in alpha testing, you can find out more info at <a href=\"https://docs.gotosocial.org\" target=\"_blank\" rel=\"nofollow noopener noreferrer\"><span class=\"invisible\">https://</span><span class=\"\">docs.gotosocial.org</span><span class=\"invisible\"></span></a></p><p>The project&#39;s souce is at <a href=\"https://github.com/superseriousbusiness/gotosocial\" target=\"_blank\" rel=\"nofollow noopener noreferrer\"><span class=\"invisible\">https://</span><span class=\"ellipsis\">github.com/superseriousbusines</span><span class=\"invisible\">s/gotosocial</span></a></p><p><a href=\"https://mastodon.online/tags/GoToSocial\" class=\"mention hashtag\" rel=\"tag\">#<span>GoToSocial</span></a> <a href=\"https://mastodon.online/tags/Fediverse\" class=\"mention hashtag\" rel=\"tag\">#<span>Fediverse</span></a> <a href=\"https://mastodon.online/tags/ActivityPub\" class=\"mention hashtag\" rel=\"tag\">#<span>ActivityPub</span></a> <a href=\"https://mastodon.online/tags/FOSS\" class=\"mention hashtag\" rel=\"tag\">#<span>FOSS</span></a> <a href=\"https://mastodon.online/tags/FLOSS\" class=\"mention hashtag\" rel=\"tag\">#<span>FLOSS</span></a> <a href=\"https://mastodon.online/tags/Libre\" class=\"mention hashtag\" rel=\"tag\">#<span>Libre</span></a> <a href=\"https://mastodon.online/tags/FreeSoftware\" class=\"mention hashtag\" rel=\"tag\">#<span>FreeSoftware</span></a> <a href=\"https://mastodon.online/tags/OpenSource\" class=\"mention hashtag\" rel=\"tag\">#<span>OpenSource</span></a> <a href=\"https://mastodon.online/tags/SelfHosting\" class=\"mention hashtag\" rel=\"tag\">#<span>SelfHosting</span></a></p>"
  },
  "attachment": [],
  "tag": [
    {
      "type": "Mention",
      "href": "https://gts.superseriousbusiness.org/users/gotosocial",
      "name": "@gotosocial@superseriousbusiness.org"
    },
    {
      "type": "Hashtag",
      "href": "https://mastodon.online/tags/selfhosting",
      "name": "#selfhosting"
    },
    {
      "type": "Hashtag",
      "href": "https://mastodon.online/tags/opensource",
      "name": "#opensource"
    },
    {
      "type": "Hashtag",
      "href": "https://mastodon.online/tags/freesoftware",
      "name": "#freesoftware"
    },
    {
      "type": "Hashtag",
      "href": "https://mastodon.online/tags/libre",
      "name": "#libre"
    },
    {
      "type": "Hashtag",
      "href": "https://mastodon.online/tags/floss",
      "name": "#floss"
    },
    {
      "type": "Hashtag",
      "href": "https://mastodon.online/tags/foss",
      "name": "#foss"
    },
    {
      "type": "Hashtag",
      "href": "https://mastodon.online/tags/activitypub",
      "name": "#activitypub"
    },
    {
      "type": "Hashtag",
      "href": "https://mastodon.online/tags/fediverse",
      "name": "#fediverse"
    },
    {
      "type": "Hashtag",
      "href": "https://mastodon.online/tags/gotosocial",
      "name": "#gotosocial"
    }
  ],
  "replies": {
    "id": "https://mastodon.online/users/FediFollows/statuses/108556046579308599/replies",
    "type": "Collection",
    "first": {
      "type": "CollectionPage",
      "next": "https://mastodon.online/users/FediFollows/statuses/108556046579308599/replies?only_other_accounts=true&page=true",
      "partOf": "https://mastodon.online/users/FediFollows/statuses/108556046579308599/replies",
      "items": []
    }
  }
}

According to the ActivityStreams spec, this is not an official type: https://www.w3.org/TR/activitystreams-vocabulary/#microsyntaxes

But since it seems to be widely adopted, we should try to support it as well.