Support LDAP login tabs wrapping to second line (and a few other login fixes)

## What does this MR do?

Fixes some regressions introduced for customers with multiple servers configured for login and/or long label names.

Also, improves styling for the login page on small screens. 

## Are there points in the code the reviewer needs to double check?

## Why was this MR needed?

See the bad behavior here: https://gitlab.com/gitlab-org/gitlab-ce/issues/23435#note_17117893

## Screenshots (if relevant)

![2016-10-19_16.34.00](/uploads/2816545b4db60ddb6430c458c2c41582/2016-10-19_16.34.00.gif)

![2016-10-19_18.07.26](/uploads/53340d5894db6441cc82b182fb47cf18/2016-10-19_18.07.26.gif)

## Does this MR meet the acceptance criteria?

- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [x] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?
https://gitlab.com/gitlab-org/gitlab-ce/issues/23435#note_17117677

See merge request !6993