Don't allow U2F set up unless an authenticator app is set up
Closes#17333
# TODO
- [ ] #17333 Authenticator should be set up before enabling U2F
- [x] Implementation
- [x] Fix/add tests
- [x] Refactor
- [x] Wait for [build](https://gitlab.com/gitlab-org/gitlab-ce/commit/964c98a3c427cac6e3de88ddc74a9f172ee9742d/builds) to pass
- [x] Assign to endboss for review
- [x] Address @DouweM's comments
- [x] No need for `javascript:void(0)`
- [x] Add screenshots
- [ ] Wait for merge
# Screenshots



See merge request !4585