Add stylelint

[puregarlic]

Like our JavaScript, we want to have uniform code style for SCSS as well. Stylelint is a versatile tool which allows us to do that. We can add it to the lint-staged command to make sure everything's compliant before committing.

[puregarlic]

Revisited this issue. Doesn't seem like there's any progress so far; the --fix flag seems to have dropped the "experimental" moniker, but it still completely overwrites the file contents. It's unclear whether or not the amount of effort required to fix stylelint issues is worth the amount of pain that it saves us, so I'm closing this issue for now.

[puregarlic]

Alright, that was quick. I've decided that the easiest way to implement Stylelint is through CodeClimate as a plugin. This way, we have our linting reports, but we can go back and fix issues when it works best for us.