add function to select a primary 2FA provider/slim down 2FA dialog

add function to select a primary 2FA provider/slim down 2FA dialog: Expected behavior

If a user has multiple 2FA providers enabled, all of them are shown on one single login screen, which might result in an overloaded login mask (check out "Current behavior" below). It might be a good idea to add a function to define a primary 2FA provider and show all alternative providers under a separate menu point.