Fix for lack of cache key for some resources

[Comandeer]

What is the purpose of this pull request?

Bug fix

Does your PR contain necessary tests?

All patches that change the editor code must include tests. You can always read more
on PR testing,
how to set the testing environment and
how to create tests
in the official CKEditor documentation.

This PR contains

• Unit tests
• Manual tests

Did you follow the CKEditor 4 code style guide?

Your code should follow the guidelines from the CKEditor 4 code style guide which helps keep the entire codebase consistent.

• PR is consistent with the code style guide

What is the proposed changelog entry for this pull request?

Fixed Issues:

* [#4761](https://github.com/ckeditor/ckeditor4/issues/4761): Fixed: not all resources loaded by the editor respect the cache key.

API Changes:

* [#4761](https://github.com/ckeditor/ckeditor4/issues/4761): [`CKEDITOR.dom.document#appendStyleSheet()`](https://ckeditor.com/docs/ckeditor4/latest/api/CKEDITOR_dom_document.html#method-appendStyleSheet) and [`CKEDITOR.tools.buildStyleHtml()`](https://ckeditor.com/docs/ckeditor4/latest/api/CKEDITOR_tools.html#method-buildStyleHtml) now use [`CKEDITOR.getUrl()`](https://ckeditor.com/docs/ckeditor4/latest/api/CKEDITOR.html#method-getUrl) to correctly handle caching of CSS files.

What changes did you make?

I've used CKEDITOR.getUrl() inside CKEDITOR.tools.buildStyleHtml() (used to load config.contentsCss files) and CKEDITOR.dom.document#appendStyleSheet() (used to add stylesheets in plugin) to add the cache key to every CSS file.

I've also changed the default value of config.contentsCss as wrapping it in CKEDITOR.getUrl() is no longer needed (not sure if we should add info about it to changelog or some notification in API docs?). I've added some basic tests for CKEDITOR.getUrl() as well – apparently it wasn't tested until now.

As it's a potentially breaking change I'll target it to major instead of master.

Which issues does your PR resolve?

Closes #4761.

[sculpt0r]

I've also changed the default value of config.contentsCss as wrapping it in CKEDITOR.getUrl() is no longer needed (not sure if we should add info about it to changelog or some notification in API docs?).

I think we don't have to mention that anywhere - since the final result will be the same.

It is nice that you covered old functionality with tests :)

I left just two small comments. Tests on building editor seems to be working fine 👍

[Comandeer]

I've applied all your suggestions. Thanks for doing the work for me ;)

[sculpt0r]

LGTM 👍 🎉

Nice job - we have another bug down :D