Clean Up SCSS/CSS files

[alexsb]

Our css is a mess. We need a way to automatically clean this up. @JackWilb can you check whether there's a reasonable, linting-based solution?

[JackWilb]

Here are some solutions we can try: https://stackoverflow.com/questions/3224363/cleaning-up-css-files

It seems like these are mostly used to find css tags that are not in use on the site, which could be a good first step.

Other than that, the best way to simplify is to reduce the amount of css we manually define. I'll take a look through and see if we can remove any CSS without consequence.