Change login-field to email-field and add translations

ref #123
Changed authentication process for admin, ref #123

 - moved unique constraint of Users identity_url to email
 - admin has nil identity_url
Removed unused method in seesion controller, ref #123
Fixed specs, ref #123
Added tests for authentication, ref #123
Fix creation of new user used by devise

ref #123
Merge B/feature#123 into B/feature#20