Multifactor authentication

[tusmester]

Implement multi-factor authentication in sensenet. Add features to the core sensenet package if possible, instead of IdentityServer.

• at least a QR code-based 2FA needs to be supported, anything else is a nice to have
• global switch for the feature in the repository
  • Disabled: no 2FA GUI, log in the user immediately
  • Optional: if enabled on user level, display the 2FA textbox AND the QR code if not configured yet
  • Forced: everybody gets the 2FA gui
• configurable per user
• new sensenet APIs for getting the settings and validating the code with and without username/pw
• admin UI: basic functionality, ON/OFF switch on a user is enough
• IdentityServer :
  • a second page after login, only if 2FA is forced either by the global or user-level switch
  • both configuring (displaying a QR code) or checking 2FA on this page
  • QR code is displayed only when the user logs in with 2FA for the first time. Subsequent requests display only the code box.
  • ~config switch for displaying the 2FA box directly on the login page~ (later)