Users can specify explicitly what name, avatar they want to use in the
config

If we don't allow segments to nest in allowsChild, Navigation.push()
does exactly what we need.

Let the homeserver choose a username for us.
This would mean:
- our random string generator does not need to take the grammar for
  mxid into account.
- we will never have a scenario in which a username is reused by
  chance.