Improve U2F/2FA flow on mobile devices

Description

After !8304 (merged) this screen is also shown on mobile devices:

Screen_Shot_2016-12-27_at_00.19.55

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