Allowing to follow entire instances / domains

This is ready for review before launching tests on the real internet between devs.

This need API V2

I think it would be great to implement this before I start working on enabeling auto-discovery of content in the network.

This Mr is dependent from this one : !1836. It should be merge before.

To test : go to /manage/moderation/domains add a domain implementing this mr, enter the domain detail page, click the follow button.

closes #762

Technical description

I added a new DomainFollow model and the associated views. It store a relation to an actor and a domain. I created activities : when a DomainFollow is created, the actor associated will follow all the public remote libraries. If a DomainFollow is deleted, all the libraryFollow associated with this follow will be deleted. This relation is created thanks to the new context_follow attribute of the LibraryFollow model. If it's set to Domain then the library follow is related to the domaiFollow and its delete with the DomainFollow deletion.

to do :

• api_serializer.DomainFollowSerializer and federation.DomainFollow
• api_views.DomainFollowViewSet
• bulk follow libraries
• create routes and activities to follow a domain
• create routes and activities to inform followers when a new library is created
• delete DomainFollow
• tests
• check if #1714 is resolved by this mr (save_remote_service_actor -> need to have the nodeinfo fetched -> add a condition) need to be rebased after !1836 is merged.

front ( to do in another mr ? ) :

• reject / accept domain follow manually ? Actually can be done through mrf policies? Need to be done in the frontend ? post to domainfollow will trigger update_follow.
• Check if update_follow is Triggered when the mrf policies change : new test ?
• Check the button Domain follow. I only pasted the libray follow button and changed libray to domain ^^'
• if the library is followed after the domain is follow, an error will occur (already folowed) -> Allow the frontend to update the follow to change follow_context from Domain to Library

Questions :

• For now every actor can follow a domain : limit to moderators ?
• I do these optimisations in this mr or later ? :
• bulk dispacth unfollow : {"type": "Undo", "objectSSSSS": {"type": "Follow"}}, context={"follow": follow}
• actors_bulk_follow_library :
• bulk routes.outbox.dispatch( {"type": "Undo", "objectSSSSS": {"type": "Follow"}}, context ={"follow": follow}
• implement a domain scan like LibraryScan ? Can be usefull if we do not receive an activity from remote.
• These optimisations do not follow activity pub standard, maybe we should only apply an exponential back-off strategy to avoid to much requests to remote servers ?