type client/user relationships

Issue Summary

We need 'typed' client/user relationships. We should have a hook that defines the allowed types, with extra modules defining their own. We should have 'admin' and 'tech' as basics (where admin can add users and do everything and tech can create sites, for example). 'billing' could be defined in a ubercart module and 'ssh' or 'files' could be available in the ITK module.

There is already code in the modules to manage the type, it's just not in the UI yet (+ we need the hooks).

This follows the work on #371769: allow for n to n and typed user/client relationships.