Support Move activities

[Gargron]

Summary

Support ActivityPub Move activities. When the object is an account (and it is the same account as the actor) and target is an account, and the target account lists the object account in its alsoKnownAs property, local followers of the object account should unfollow that account and follow the new one automatically.

[nbsp]

for reference, this is how Pleroma respects incoming Move activities:

• there is a user-level toggle in the settings, to allow auto-follow when following account moves
• if enabled, when an account moves, it auto-follows the new one and sends the user a notification that the account has been migrated. as far as I can tell Pleroma does not unfollow the original account

[Gargron]

The reason for unfollowing the old account is to dissuade people from farming and then selling followers. If you do not unfollow the old account, the original owner can keep sending out Move activities to different accounts.

[robflop]

That's a good idea, and a feature I've seen people talk about wanting a few times already.

Logically speaking, successive moves should form a chain of moves from accounts where there is never a pair of successive moves from the same account in a row, so I believe the unfollow restriction handles that well.

[tamaina]

Duplicate of #5475