Add users

[half-ogre]

The following actions should only be available to approved users:

• starting a build manually (i.e., not through a web hook)
• building a PR (for security reasons)
• re-building a failed build

Anyone should be able to view the home page, /builds, and /builds/{id}.

To-Do (v0.1)

• [x] #55: let anyone sign up (i.e., create a user account)
• [x] #56: let users sign in
• [x] #57: let users sign out
• [x] #59: let administrators make other users administrators
• [x] #58: only let administrators start builds
• [x] #66: Require an initial administrator

  To-Do (vLater)

• [ ] require email confirmation
• [ ] let users reset their password via email
• [ ] let users change their password
• [ ] let administrators invite new users