Rename Activitypub to ActivityPub

* Fix nil input not handled well in AuthorExtractor concern

* Fix hard error in ProcessFeedService when replied-to status has been deleted

* Fix nil errors in ProcessInteractionService when favourited status
cannot be found

Fix #2196 - Respond with 201 when Salmon accepted, 400 when unverified
Fix #2629 - Correctly handle confirm_domain? for local accounts
Unify rules for extracting author acct from XML, prefer <email>, fall back
to <name> + <uri> (see also #2017, #2172)