Remove User.upgrade_changeset in favor of remote_user_creation

The two changesets had the same purpose, yet some changes were updated in one, but not the other (uri, for example).

Also makes Transmogrifier.upgrade_user_from_ap_id be called from ActivityPub.make_user_from_ap_id only when the user is actually not AP enabled yet.

I did not bother rewriting tests that used User.insert_or_update to use the changeset instead because they seemed to just test the implementation, rather than behavior.