* Remove Salmon and PubSubHubbub endpoints

* Add error when trying to follow OStatus accounts

* Fix new accounts not being created in ResolveAccountService

* ActivityPub migration procedure

Once one account is detected as going from OStatus to ActivityPub,
invalidate WebFinger cache for other accounts from the same domain

* Unsubscribe from PuSH updates once we receive an ActivityPub payload

* Re-subscribe to PuSH unless already unsubscribed, regardless of protocol