Also, fixes crashes when entering tag and threads due to me forgetting to implement the interfaces required by the code that removes posts from timelines when blocking/muting.
Also fixes a small bug where for mentions of users from other instances, clicking on the mention would open the profile in the browser instead of in-app.
Mentions are also, incidentally, but still link to the account page for that user in the browser. This should be changed to an in-app account page when that's finished, but it's actually fairly suitable fallback behaviour for now.
