Provide simple user registration with basic approval/quota scheme

[itzg]

This takes the first step towards #29 by:

• Allow an unauthenticated user to register for a new account
• Don't allow new accounts to login until an admin has approved their account
• Send an email to an admin address when a new user has registered an account
• All non-admin accounts will have a system-wide configured quota (such as 3 containers)
• Admins need a way to watch for high CPU containers of all users and stop them