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/controllers/activitypub/inboxes_controller.rb

@@ -0,0 +1,30 @@
+# frozen_string_literal: true
+
+class ActivityPub::InboxesController < Api::BaseController
+  include SignatureVerification
+
+  before_action :set_account
+
+  def create
+    if signed_request_account
+      process_payload
+      head 201
+    else
+      head 202
+    end
+  end
+
+  private
+
+  def set_account
+    @account = Account.find_local!(params[:account_username])
+  end
+
+  def body
+    @body ||= request.body.read
+  end
+
+  def process_payload
+    ActivityPub::ProcessingWorker.perform_async(signed_request_account.id, body.force_encoding('UTF-8'))
+  end
+end