[performance/chore] Update local + federated (public) timelines to use internal/timeline

[tsmethurst]

Currently when a user requests the local / public timeline via API, we go straight to making database calls and filtering items out that don't apply.

This works more or less OK, but can cause performance issues (lots of database calls) and issues where all items get filtered out and the user gets an empty timeline back (#2410, #2273).

We mostly mitigated this in #2784, but the issue still crops up again occasionally.

If we were to store new statuses eligible for public / local timelining as they come in, using a timeline from the internal/timeline package, we could speed up serving timelines, and also make it easier to avoid filtering out every entry.