Add ActivityPub inbox (#4216)

* Add ActivityPub inbox

* Handle ActivityPub deletes

* Handle ActivityPub creates

* Handle ActivityPub announces

* Stubs for handling all activities that need to be handled

* Add ActivityPub actor resolving

* Handle conversation URI passing in ActivityPub

* Handle content language in ActivityPub

* Send accept header when fetching actor, handle JSON parse errors

* Test for ActivityPub::FetchRemoteAccountService

* Handle public key and icon/image when embedded/as array/as resolvable URI

* Implement ActivityPub::FetchRemoteStatusService

* Add stubs for more interactions

* Undo activities implemented

* Handle out of order activities

* Hook up ActivityPub to ResolveRemoteAccountService, handle
Update Account activities

* Add fragment IDs to all transient activity serializers

* Add tests and fixes

* Add stubs for missing tests

* Add more tests

* Add more tests

app/workers/activitypub/processing_worker.rb

@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+class ActivityPub::ProcessingWorker
+  include Sidekiq::Worker
+
+  sidekiq_options backtrace: true
+
+  def perform(account_id, body)
+    ProcessCollectionService.new.call(body, Account.find(account_id))
+  end
+end

app/workers/activitypub/thread_resolve_worker.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+class ActivityPub::ThreadResolveWorker
+  include Sidekiq::Worker
+
+  sidekiq_options queue: 'pull', retry: false
+
+  def perform(child_status_id, parent_uri)
+    child_status  = Status.find(child_status_id)
+    parent_status = ActivityPub::FetchRemoteStatusService.new.call(parent_uri)
+
+    return if parent_status.nil?
+
+    child_status.thread = parent_status
+    child_status.save!
+  end
+end