Replace jquery-cookie plugin with js-cookie
## What does this MR do?
Removes an outdated and unmaintained dependency [jquery.cookie](https://github.com/carhartl/jquery-cookie) with its successor [js.cookie](https://github.com/js-cookie/js-cookie) which is not dependent on jQuery.
Also this MR adds a global default cookie `path` setting which defaults to the GitLab root directory. Now, setting a cookie to any path other than `gon.relative_url_root` requires an explicit opt-out. This is a sensible default and should prevent accidental omission going forward.
## Are there points in the code the reviewer needs to double check?
Double check that all instances of `$.cookie(x)` have been replaced by `Cookies.get(x)` and all instances of `$.cookie(x, y, z)` are replaced with `Cookies.set(x, y, z)`. All javascripts which set or retrieve cookie values should function just as before.
## Why was this MR needed?
One more step toward shaking off jQuery as a dependency (#23022), also [jquery.cookie](https://github.com/carhartl/jquery-cookie) is no longer maintained and hasn't seen an update in 2+ years.
## Does this MR meet the acceptance criteria?
- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG.md) entry added
- Tests
- [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?
#23022
See merge request !7085
052480ae53