2nd factor login wishlist

[phorward]

This is a wishlist from previous viur-meeting regarding second factor login:

• Further second factors (#820 targets a selection of several second factors)
  • Authy/Google Authenticator (#578 focuses on this)
  • SMS
  • E-Mail
  • YubiKey
• TAN-like list of one time tokens as security fallback (like with GitHub)
• Standardised self-service for users
  • Manage multiple different factors. Let the user choose which of his configured 2nd factor he wants to use for the current login.
  • User-Status "2nd factor required" => Login only to configure second factor

Issue #802 and #800 also partly standardize the second-factor providers.

[sveneberth]

I just saw that I had already created a issue earlier. So this is actually a duplicate of #795.