Allow user to switch 2FA mechanism if more than one is supported

Add links 'use a numeric / hardware token instead' to the 2FA login
pages, using the list of available 2FA mechanisms found in the
auth Response.
3 jobs for switch-2fa in 2 minutes and 43 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build Pkgsrc
passed #7636
build_pkgsrc:stretch

00:00:15

 
  Build Pkg
passed #7637
amd64
build_pkg:stretch:amd64

00:00:57

passed #7638
arm64
build_pkg:stretch:arm64

00:02:26