feature: external resources auto-follow and unfollow local ones on activity reception

Closes #268 (closed)

This turned out to be a little more complex than I expected

Currently, when the sender sends me an activity, they will also create a follower for me, so that if their local resource is changed, I am notified of it

Now, when I receive the activity, I will also create a follower for them. As Add, Create or Update could receive a large nested object (in theory), this means that in practice any local object will be followed by its external parent, e.g.:

{
'id': external,
'type': 'circle-member',
'circle': {
   'id': external # no follower is created
},
'user': {
   'id': local # the external **circle-member** is set to follow the local **user**
}
}

When I receive a Remove or Delete Activity, I will delete the followers as expected