Two-factor authentication

Implement's Two-factor authentication using tokens.

• Authentication logic
• Enable/disable 2FA feature
• Make 2-step login process if 2FA enabled
• Backup codes
• Backup code removed after being used
• Check backup codes for mysql db (mention mysql limitation if applied)
• Add tests
• Test if https://github.com/tinfoil/devise-two-factor#disabling-automatic-login-after-password-resets applies, and address if so
• 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