Two-factor authentication

Implement's Two-factor authentication using tokens.

- [X] Authentication logic
- [X] Enable/disable 2FA feature
- [x] Make 2-step login process if 2FA enabled
- [x] Backup codes
- [x] Backup code removed after being used
- [x] Check backup codes for mysql db (mention mysql limitation if applied)
- [x] Add tests
- [x] Test if https://github.com/tinfoil/devise-two-factor#disabling-automatic-login-after-password-resets applies, and address if so
- [x] Wait for fixed version of `attr_encrypted` or fork and use forked version - https://github.com/attr-encrypted/attr_encrypted/issues/155

Fixes http://feedback.gitlab.com/forums/176466-general/suggestions/4516817-implement-two-factor-authentication-2fa

See merge request !474