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

![Screen_Shot_2016-06-15_at_8.18.03_AM](/uploads/26531fa7f6e5d7617fd11d1779021b4f/Screen_Shot_2016-06-15_at_8.18.03_AM.png)
![Screen_Shot_2016-06-15_at_8.18.37_AM](/uploads/ceaae97a987a15d3e04dd76aa8a944bd/Screen_Shot_2016-06-15_at_8.18.37_AM.png)
![Screen_Shot_2016-06-15_at_8.18.47_AM](/uploads/394224d5fcff759d5acc3bf39a138530/Screen_Shot_2016-06-15_at_8.18.47_AM.png)

See merge request !4585