Improve ActivityPub representations (#3844)

* Improve webfinger templates and make tests more flexible

* Clean up AS2 representation of actor

* Refactor outbox

* Create activities representation

* Add representations of followers/following collections, do not redirect /users/:username route if format is empty

* Remove unused translations

* ActivityPub endpoint for single statuses, add ActivityPub::TagManager for better
URL/URI generation

* Add ActivityPub::TagManager#to

* Represent all attachments as Document instead of Image/Video specifically
(Because for remote ones we may not know for sure)

Add mentions and hashtags representation to AP notes

* Add AP-resolvable hashtag URIs

* Use ActiveModelSerializers for ActivityPub

* Clean up unused translations

* Separate route for object and activity

* Adjust cc/to matrices

* Add to/cc to activities, ensure announce activity embeds target status and
not the wrapper status, add "id" to all collections

config/locales/en.yml

@@ -44,15 +44,6 @@ en:
     remote_follow: Remote follow
     reserved_username: The username is reserved
     unfollow: Unfollow
-  activitypub:
-    activity:
-      announce:
-        name: "%{account_name} shared an activity."
-      create:
-        name: "%{account_name} created a note."
-    outbox:
-      name: "%{account_name}'s Outbox"
-      summary: A collection of activities from user %{account_name}.
   admin:
     accounts:
       are_you_sure: Are you sure?