ActivityPub delivery (#4566)

* Deliver ActivityPub Like

* Deliver ActivityPub Undo-Like

* Deliver ActivityPub Create/Announce activities

* Deliver ActivityPub creates from mentions

* Deliver ActivityPub Block/Undo-Block

* Deliver ActivityPub Accept/Reject-Follow

* Deliver ActivityPub Undo-Follow

* Deliver ActivityPub Follow

* Deliver ActivityPub Delete activities

Incidentally fix #889

* Adjust BatchedRemoveStatusService for ActivityPub

* Add tests for ActivityPub workers

* Add tests for FollowService

* Add tests for FavouriteService, UnfollowService and PostStatusService

* Add tests for ReblogService, BlockService, UnblockService, ProcessMentionsService

* Add tests for AuthorizeFollowService, RejectFollowService, RemoveStatusService

* Add tests for BatchedRemoveStatusService

* Deliver updates to a local account to ActivityPub followers

* Minor adjustments

app/workers/activitypub/distribution_worker.rb

@@ -0,0 +1,38 @@
+# frozen_string_literal: true
+
+class ActivityPub::DistributionWorker
+  include Sidekiq::Worker
+
+  sidekiq_options queue: 'push'
+
+  def perform(status_id)
+    @status  = Status.find(status_id)
+    @account = @status.account
+
+    return if skip_distribution?
+
+    ActivityPub::DeliveryWorker.push_bulk(inboxes) do |inbox_url|
+      [payload, @account.id, inbox_url]
+    end
+  rescue ActiveRecord::RecordNotFound
+    true
+  end
+
+  private
+
+  def skip_distribution?
+    @status.direct_visibility?
+  end
+
+  def inboxes
+    @inboxes ||= @account.followers.inboxes
+  end
+
+  def payload
+    @payload ||= ActiveModelSerializers::SerializableResource.new(
+      @status,
+      serializer: ActivityPub::ActivitySerializer,
+      adapter: ActivityPub::Adapter
+    ).to_json
+  end
+end