Explicitly declare all javascript globals and all eslint rule violations
This merge request takes every single external global variable referenced within a javascript file and explicitly marks it with a `/* global Foo */` comment block at the top of the script.
This also expands all blanket instances of `/* eslint-disable */` with an explicit list of disabled rules. This is useful because if we need to search for violations of a particular rule we can simply grep the codebase for something like `no-unused-vars` or `semi` and find all of the places where this rule has yet to be fixed.
Lastly, it also removes and resolves any existing `no-undef` eslint violations. This is useful for catching mistakes like forgetting to declare a variable with `var`/`let`/`const` which can cause hard to find bugs.
## What does this MR do?
1. Looks for generic uses of `/* eslint-disable */` and refactors them into individual rule exceptions.
2. Looks for all occurrences of `/* eslint-disable ... no-undef */` and resolves them by either fixing bugs or declaring globals with `/* global Foo */`.
## Are there points in the code the reviewer needs to double check?
This touches a lot of files, most changes touch nothing other than comment blocks or whitespace. The exceptions are the following 14 files which required some small bug fixes after removing `no-undef`:
- api.js
- breakpoints.js
- build.js
- commits.js
- diff_notes/components/jump_to_discussion.js.es6
- gfm_auto_complete.js.es6
- gl_dropdown.js
- groups_select.js
- importer_status.js
- namespace_select.js
- notes.js
- preview_markdown.js
- projects_list.js
- single_file_diff.js
## Why was this MR needed?
Removal of ~"technical debt" and some necessary changes to help !7288
## Screenshots (if relevant)
N/A
## Does this MR meet the acceptance criteria?
- [ ] ~~[Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) added~~
- [ ] ~~[Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)~~
- [ ] ~~API support added~~
- Tests
- [ ] ~~Added for this feature/bug~~
- [ ] 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?
N/A
See merge request !8043
- Tab between register and sign in forms
- Add individual input validation error messages
- Validate username
- Update many styles for all login-box forms
