Implements iChain authentication

These features are necessary to run OSEM inside the Suse authentication system!
Attention: You have to copy the config example!
The problem here is that if we activate iChain there are no sign_in, register etc. routes anymore!
I've copied this from devise HowTo
https://github.com/plataformatec/devise/wiki/How-To:-Redirect-back-to-current-page-after-sign-in,-sign-out,-sign-up,-update#code