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/ja.yml

@@ -30,15 +30,6 @@ ja:
     remote_follow: リモートフォロー
     reserved_username: このユーザー名は予約されています。
     unfollow: フォロー解除
-  activitypub:
-    activity:
-      announce:
-        name: "%{account_name} さんがアクティビティをシェアしました"
-      create:
-        name: "%{account_name} さんがノートを作成しました"
-    outbox:
-      name: "%{account_name} さんの送信トレイ"
-      summary: "%{account_name} さんからのアクティビティコレクション"
   admin:
     accounts:
       are_you_sure: 本当に実行しますか?