'Returns whether a user can see another user\'s presence.'

@defer.inlineCallbacks
def is_visible(self, observed_user, observer_user):

    observer_room_ids = (yield self.store.get_rooms_for_user(observer_user.to_string()))
    observed_room_ids = (yield self.store.get_rooms_for_user(observed_user.to_string()))
    if (observer_room_ids & observed_room_ids):
        defer.returnValue(True)
    accepted_observers = (yield self.store.get_presence_list_observers_accepted(observed_user.to_string()))
    defer.returnValue((observer_user.to_string() in accepted_observers))
