* Move ActivityPub::FetchRemoteAccountService to ActivityPub::FetchRemoteActorService
ActivityPub::FetchRemoteAccountService is kept as a wrapper for when the actor is
specifically required to be an Account
* Refactor SignatureVerification to allow non-Account actors
* fixup! Move ActivityPub::FetchRemoteAccountService to ActivityPub::FetchRemoteActorService
* Refactor ActivityPub::FetchRemoteKeyService to potentially return non-Account actors
* Refactor inbound ActivityPub payload processing to accept non-Account actors
* Refactor inbound ActivityPub processing to accept activities relayed through non-Account
* Refactor how Account key URIs are built
* Refactor Request and drop unused key_id_format parameter
* Rename ActivityPub::Dereferencer `signature_account` to `signature_actor`
Only look up private toots from database if the request failed because of 401,
403 or 404 errors, as those may indicate a private toot, rather than something
that isn't a toot or cannot be processed.
* Remove “protocol” argument and return value, as only ActivityPub is supported
* Remove FetchRemoteAccountService, only use ActivityPub::FetchRemoteAccountService
* Fix tests
