* Do not raise unretryable exceptions in ResolveRemoteAccountService

* Removed fatal exceptions from ResolveRemoteAccountService

Exceptions that cannot be retried should not be raised. New exception
class for those that can be retried (Mastodon::UnexpectedResponseError)

* Refactor ResolveRemoteAccountService

* Remove trailing whitespace

* Use redis locks around critical ResolveRemoteAccountService code

* Add test for race condition of lock

Rename Activitypub to ActivityPub

* Instead of parsing shared status contents verbatim, make roundtrip
to purported original URL. Confirm that the "original" URL is from the
same domain as the author it claims to be from.

* Fix obvious typo, add comment

* Use URI look-up first

* Add test, update Goldfinger dependency to make less useless HTTP requests per Webfinger lookup

* Fix possibility of unrightful webfinger redirect

* Add more tests for FollowRemoteAccountService