Support Mastodon 3.0 "Move" activity behavior
Mastodon 3.0 tries to move followers over when it receives a 'Move' message.
How this works according to gargron:
Moveactivity from origin account, check if
targetaccount has origin account in
alsoKnownAs, if yes, follow
target, if successful, unfollow origin
While this will always be pretty hit-or-miss, I think it wouldn't hurt to support it.