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/lib/activitypub/adapter.rb

@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+class ActivityPub::Adapter < ActiveModelSerializers::Adapter::Base
+  def self.default_key_transform
+    :camel_lower
+  end
+
+  def serializable_hash(options = nil)
+    options = serialization_options(options)
+    serialized_hash = { '@context': 'https://www.w3.org/ns/activitystreams' }.merge(ActiveModelSerializers::Adapter::Attributes.new(serializer, instance_options).serializable_hash(options))
+    self.class.transform_key_casing!(serialized_hash, instance_options)
+  end
+end

app/lib/activitypub/tag_manager.rb

@@ -0,0 +1,69 @@
+# frozen_string_literal: true
+
+require 'singleton'
+
+class ActivityPub::TagManager
+  include Singleton
+  include RoutingHelper
+
+  COLLECTIONS = {
+    public: 'https://www.w3.org/ns/activitystreams#Public',
+  }.freeze
+
+  def url_for(target)
+    return target.url if target.respond_to?(:local?) && !target.local?
+
+    case target.object_type
+    when :person
+      short_account_url(target)
+    when :note, :comment, :activity
+      short_account_status_url(target.account, target)
+    end
+  end
+
+  def uri_for(target)
+    return target.uri if target.respond_to?(:local?) && !target.local?
+
+    case target.object_type
+    when :person
+      account_url(target)
+    when :note, :comment, :activity
+      account_status_url(target.account, target)
+    end
+  end
+
+  # Primary audience of a status
+  # Public statuses go out to primarily the public collection
+  # Unlisted and private statuses go out primarily to the followers collection
+  # Others go out only to the people they mention
+  def to(status)
+    case status.visibility
+    when 'public'
+      [COLLECTIONS[:public]]
+    when 'unlisted', 'private'
+      [account_followers_url(status.account)]
+    when 'direct'
+      status.mentions.map { |mention| uri_for(mention.account) }
+    end
+  end
+
+  # Secondary audience of a status
+  # Public statuses go out to followers as well
+  # Unlisted statuses go to the public as well
+  # Both of those and private statuses also go to the people mentioned in them
+  # Direct ones don't have a secondary audience
+  def cc(status)
+    cc = []
+
+    case status.visibility
+    when 'public'
+      cc << account_followers_url(status.account)
+    when 'unlisted'
+      cc << COLLECTIONS[:public]
+    end
+
+    cc.concat(status.mentions.map { |mention| uri_for(mention.account) }) unless status.direct_visibility?
+
+    cc
+  end
+end