Ensure all javascripts use GitLab install directory as the cookie path.

## What does this MR do?

Fixes #20435 and standardizes all JavaScript-set cookies to use the same root url.

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

Nothing I can think of.  This MR is pretty straight forward.

## Why was this MR needed?

Cookie paths were inconsistent across front-end scripts.  Some would default to '/' and others had no set path and would end up tied to whatever page they were set on.  Also as @connorshea noted, we cannot assume GitLab is installed at '/'.

Before:

![Screen_Shot_2016-08-02_at_10.01.18_AM](/uploads/fa0238e2edf67a8e8ff48f8ee9c8d955/Screen_Shot_2016-08-02_at_10.01.18_AM.png)

After:

![Screen_Shot_2016-08-02_at_10.03.03_AM](/uploads/f1b95936d6f95f2ef1d7db10b5fb09d1/Screen_Shot_2016-08-02_at_10.03.03_AM.png)


## What are the relevant issue numbers?

#20435

## Screenshots (if relevant)

see above

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- Tests
  - [x] Added for this feature/bug
  - [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 !5627