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

app/serializers/activitypub/note_serializer.rb

@@ -0,0 +1,106 @@
+# frozen_string_literal: true
+
+class ActivityPub::NoteSerializer < ActiveModel::Serializer
+  attributes :id, :type, :summary, :content,
+             :in_reply_to, :published, :url,
+             :actor, :to, :cc, :sensitive
+
+  has_many :media_attachments, key: :attachment
+  has_many :virtual_tags, key: :tag
+
+  def id
+    ActivityPub::TagManager.instance.uri_for(object)
+  end
+
+  def type
+    'Note'
+  end
+
+  def summary
+    object.spoiler_text.presence
+  end
+
+  def content
+    Formatter.instance.format(object)
+  end
+
+  def in_reply_to
+    ActivityPub::TagManager.instance.uri_for(object.thread) if object.reply?
+  end
+
+  def published
+    object.created_at.iso8601
+  end
+
+  def url
+    ActivityPub::TagManager.instance.url_for(object)
+  end
+
+  def actor
+    ActivityPub::TagManager.instance.uri_for(object.account)
+  end
+
+  def to
+    ActivityPub::TagManager.instance.to(object)
+  end
+
+  def cc
+    ActivityPub::TagManager.instance.cc(object)
+  end
+
+  def virtual_tags
+    object.mentions + object.tags
+  end
+
+  class MediaAttachmentSerializer < ActiveModel::Serializer
+    include RoutingHelper
+
+    attributes :type, :media_type, :url
+
+    def type
+      'Document'
+    end
+
+    def media_type
+      object.file_content_type
+    end
+
+    def url
+      object.local? ? full_asset_url(object.file.url(:original, false)) : object.remote_url
+    end
+  end
+
+  class MentionSerializer < ActiveModel::Serializer
+    attributes :type, :href, :name
+
+    def type
+      'Mention'
+    end
+
+    def href
+      ActivityPub::TagManager.instance.uri_for(object.account)
+    end
+
+    def name
+      "@#{object.account.acct}"
+    end
+  end
+
+  class TagSerializer < ActiveModel::Serializer
+    include RoutingHelper
+
+    attributes :type, :href, :name
+
+    def type
+      'Hashtag'
+    end
+
+    def href
+      tag_url(object)
+    end
+
+    def name
+      "##{object.name}"
+    end
+  end
+end