Add rotp gem

https://github.com/mdp/rotp
Added model for two-factor authenticators
Processor for two-factor authenticator overview
Fixed migration
Two-factor authenticator overview
Only include active two-factor authenticators
TwoFactorAuthenticatorProvider
Default settings
Cleanup method for two-factor authenticators
Fixed documentation
Skeleton for TwoFactorAuthenticatorActivator
Minor wording changes
Extracted ValidationResult
Allow activation of tow-factor authenticators
Add flag for pendig authentication
Allow activating ticket-granting tickets with pending two-factor
Refactoring
Removed unused parameter
Documentation
Don't store pending tgt in a cookie