Feature request: Allow users to create subteams under their own name. (username.subteam)

I tried to create team `ponies` but that name was already taken. Bummer!
This gave me a great idea, though:

Currently, users and top-level teams share the same namespace, and I think that's fine.

However, **users should clearly be able to create subteams under their own name**.
Said another way, users should be able to use their name as a subteam root.

For instance, as user `hexstream`, I should be able to create a subteam `hexstream.ponies`.
(Needless to say, nobody else than me should be able to create subteams under my name.)

(Of course I could mangle this name in whatever ugly way to create it as a top-level team, but this would be very inelegant, and it would reserve that name forever, since old user or team names can't be reused.)

No files, tests, or entry points are named in the issue. Start by locating the team-creation and namespace-handling entry points, then determine how ownership and name reuse are enforced; done means users can create username.subteam names while other users cannot create subteams under that username.

Written by the indexing model from the issue text.