CSS style guide

Working towards what was discussed in #13552, this adds the [SCSS Linter gem](https://github.com/brigade/scss-lint) for style guide conformity in CI.

TODO:
- [x] Agree on and write SCSS Style Guide Documentation.
- [x] Document the `scss-lint` config file.
- [x] Figure out how best to run this in CI, right now it's taking longer than I would think it should.
- [x] Use CSSComb for auto-correction (Maybe just include a CSSComb config file and have developers run the node package manually if they're interested in using it?).

My logic for not using CSSComb in the first place is that, AFAIK, we don't currently require Node/NPM, even in the dev environment. Maybe I'm wrong about that? `scss-lint` is a Ruby implementation of a similar concept, but it doesn't include autocorrect.

Is there a way we can run get GitLab CI to run `scss-lint` without having to put together the full GitLab application? Seeing as it's just static analysis, it seems like a waste of time/resources.

/cc @jschatz1 @rspeicher

See merge request !3069