Merge branch 'add-vertical-scrolling-sub-nav' into 'master'

Add horizontal scrolling to all sub-navs on mobile viewports

## What does this MR do?
Adds horizontal scrolling to all the sub navigation on mobile views. This was previously only available on the repository tab (commits page). 

## Are there points in the code the reviewer needs to double check?
Perhaps a better way to refactor out the common haml code

## Why was this MR needed?
Adding it to all to keep the UI consistency and to offer mobile users a better interface

## What are the relevant issue numbers?
Closes #20779 

## Screenshots (if relevant)
Before:
![Screen_Shot_2016-08-23_at_3.52.35_PM](/uploads/3833da4252cf616266a77654cee89faf/Screen_Shot_2016-08-23_at_3.52.35_PM.png)

After:
![Screen_Shot_2016-08-23_at_3.52.21_PM](/uploads/5a023bb1e639f16f2471009dd6ae0d62/Screen_Shot_2016-08-23_at_3.52.21_PM.png)

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- Tests
  - [x] All builds are passing
- [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 you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5762

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