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.