core: method to know if a profile/entity is an admin: the new `is_admin` SatXMPPEntity property tell if it's an administrator. Admin JIDs are retrieve on init, so they can be looked after when profle is not available (notably when a component handle a request and has only a JID available). The new `memory.isAdminJID` method is then used.

[DEFAULT]
media_dir = /src/libervia-media