Add automagical AS Publicised Group(s)

via registration file "users" namespace:

```YAML
...
namespaces:
  users:
    - exclusive: true
      regex: '.*luke.*'
      group_id: '+all_the_lukes:hsdomain'
...
```

This is part of giving App Services their own groups for matching users. With this, ghost users will be given the appeareance that they are in a group and that they have publicised the fact, but _only_ from the perspective of the `get_publicised_groups_for_user` API.

synapse/handlers/groups_local.py

@@ -375,6 +375,12 @@ class GroupsLocalHandler(object):
     def get_publicised_groups_for_user(self, user_id):
         if self.hs.is_mine_id(user_id):
             result = yield self.store.get_publicised_groups_for_user(user_id)
+
+            # Check AS associated groups for this user - this depends on the
+            # RegExps in the AS registration file (under `users`)
+            for app_service in self.store.get_app_services():
+                result.extend(app_service.get_groups_for_user(user_id))
+
             defer.returnValue({"groups": result})
         else:
             result = yield self.transport_client.get_publicised_groups_for_user(