Refactored less readable existance checks

## What does this MR do?
Clean up complex JavaScript expressions generated by conditional assignment and existence checks in CoffeeScript.
These are small changes that effect 1 or 2 lines at a time, but should make sections of code more readable.

## Are there points in the code the reviewer needs to double check?
Changes in [gl_dropdown.js](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/assets/javascripts/gl_dropdown.js) don't do much (largely just splitting stuff onto multiple lines). Is this a bit pointless?

## Why was this MR needed?
Auto-generated JS contains complex expressions that will be difficult to work with in future.

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [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 you do - 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?

A child issue of #21887 
Closes #21988

See merge request !6289