Add the possibility to define some related fediverse instances in administration

The idea is to be able to have an admin list of predefined instances in the ActivityPub administration.
Those services would be defined by a name, an URL to access them, maybe a type and a logo.
It could be used to discover directly users in those instances if webfinger is not available for example. It could also be used to display a logo next to a notif to tell about which instance it is.
We could also improve it later to allow following an entire XWiki instance.
And we could also improve it to only allow the interaction (follow of users etc) on the list of instances.