Add users

This is the first part of adding users to QED. It:

- [ ] adds a sign-up form (fixes #55)
- [ ] adds a sign-in form (fixes #56)
- [ ] adds a sign-out button (fixes #57)
- [ ] restricts starting builds to administrators (fixes #58)
- [ ] adds a users page where administrators can grant and revoke administrative privilege (fixes #59)