Allow admin to create tokens for other users

[mdekstrand]

I was trying to set up read-only role users (e.g. for deployment) that can check out via HTTP with authentication tokens, but could only create tokens for a new user after assigning them a public key and connecting as them.

It would be convenient if an admin user could create auth tokens for other users. Then I could easily create as many role accounts and authentication tokens as I need from my primary user. A user token create command, or a --user option to token create.

Analogous list / delete etc. capabilities could also be useful, so admin can revoke compromised tokens without needing to directly edit the database.