Allow importing scss files in JS

What does this MR do?

1. Add sass-loader to allow importing SCSS into JS
2. Fix .css regex in webpack config for CSS files

Was needed to import some colors from variables.scss in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/7538#note_104940229

With this MR we can export vars from SCSS and use them.

// variables.scss
$red: #f00;

:export {
  red: $red;
}

// something.js
import { red } from './variables.scss';

console.log(red) // #f00

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides