Improve U2F/2FA flow on mobile devices
Description
After !8304 (merged) this screen is also shown on mobile devices:
However, you cannot plug in a U2F device on a smartphone.
Proposal
It would be better to directly show the 2FA code input field on mobile.
Links / references
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8304#note_22738247