Deshabilitar usuarios

- [x] Confirmación de usuarios: https://www.pivotaltracker.com/story/show/56405138 (falta mejorar la thank you page de confirmar usuario)
- [x] Inhabilitar usuarios: https://www.pivotaltracker.com/story/show/56405140
- [x] Sign in de Admins: https://www.pivotaltracker.com/story/show/56405162