Attribute artist

See #689 (closed)

This MR implement the federation part for metadata update:

• Broadcasting applied edits to federation (via an Update activity)
• Receiving and handling those updates
• Verifying that the update comes either from the service actor or the actor referenced in the attributedTo field of the updated entity

This requests also restore some tests that were previously deleted in federation/tests_serializers.py

Cf https://mastodon.eliotberriot.com/@eliotberriot/101902286936717693

Peek_2019-04-10_16-06