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/delivery_worker.rb

@@ -0,0 +1,37 @@
+# frozen_string_literal: true
+
+class ActivityPub::DeliveryWorker
+  include Sidekiq::Worker
+
+  sidekiq_options queue: 'push', retry: 5, dead: false
+
+  HEADERS = { 'Content-Type' => 'application/activity+json' }.freeze
+
+  def perform(json, source_account_id, inbox_url)
+    @json           = json
+    @source_account = Account.find(source_account_id)
+    @inbox_url      = inbox_url
+
+    perform_request
+
+    raise Mastodon::UnexpectedResponseError, @response unless response_successful?
+  rescue => e
+    raise e.class, "Delivery failed for #{inbox_url}: #{e.message}"
+  end
+
+  private
+
+  def build_request
+    request = Request.new(:post, @inbox_url, body: @json)
+    request.on_behalf_of(@source_account, :uri)
+    request.add_headers(HEADERS)
+  end
+
+  def perform_request
+    @response = build_request.perform
+  end
+
+  def response_successful?
+    @response.code > 199 && @response.code < 300
+  end
+end