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.