Refactor remnants of CoffeeScript destructured opts and super into ES6

What does this MR do?

Gets rid of some CoffeeScript remnants that are difficult to work with, and updates them to use ES6 syntax/constructs. I updated nearby code to ES6, but these updates usually just included using the es6 class syntax and scoped variable keywords.

Important: For the most part, I tried to avoid refactoring the design of the existing code. If I attempted that, this would've taken much much longer and it would've been out of this issue's scope.

Why was this MR needed?

The compiled coffeescript for destructured default params is very difficult to work in.

These changes makes some of these rough patches more maintainable:

e.g.

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Meta Issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/21887

Child Issues:

https://gitlab.com/gitlab-org/gitlab-ce/issues/21942

https://gitlab.com/gitlab-org/gitlab-ce/issues/21941

https://gitlab.com/gitlab-org/gitlab-ce/issues/21943

To Test Manually:

app/assets/javascripts/LabelManager.js.es6 -> diff

• Navigate to Labels and click around (toggle label priority, add new labels, subcribe, edit, delete)

app/assets/javascripts/blob/blob_ci_yaml.js.es6 -> diff

• Create new file and give it the name .gitlab-ci.yml, then attempt to select a template

app/assets/javascripts/blob/blob_license_selectors.js.es6 -> diff

• Create new file and give it the name license, then attempt to select a template

app/assets/javascripts/blob/template_selector.js.es6 -> diff

• This class is extended by the two previously mentioned files. As long as they work, this should work.

app/assets/javascripts/issues-bulk-assignment.js.es6 -> diff

• Visit issues, assign multiple issues a new label or milestone and update.

app/assets/javascripts/profile/gl_crop.js.es6 -> diff

• Visit your profile settings, upload a photo and attempt to crop and set as new profile picture.

app/assets/javascripts/profile/profile.js.es6 > diff

• Visit your profile settings, change various fields (Name, Bio, Email, Notifs settings) and submit form

app/assets/javascripts/todos.js.es6 -> diff

• Visit an issue and Add Todo, then visit Todos. Try out the sorting features, making sure everything looks right. Mark the Todo done.

app/assets/javascripts/user.js.es6 -> diff

• Visit user profile and click between tabs.

app/assets/javascripts/user_tabs.js.es6 -> diff

• Same as previous.

cc: @jschatz1