[feature] Allow an option for users to automatically reject follow requests

[tsmethurst]

We currently have functionality to automatically approve follow requests. And we have the default manuallyApprovesFollowers mode that puts follow requests in a queue for review. It might be nice to have functionality that automatically rejects follow requests (unless the account receiving the follow request already follows the requester).

This would allow people overwhelmed by a barrage of follow requests to just reject them all without having them added to an ever-growing queue/backlog that they'd need to look through later. Could be a useful anti-spam feature too, if spammers ever decide to use follow request spamming as a vector.