Add CSS autoprefixing

[stvnrlly]

As far as we know, this is only necessary to get the chevron image in the banner to work.

I'm trying to avoid adding a gulp pipeline here, but that shifts complexity elsewhere. I think that's preferable, as adding Jekyll plugins doesn't require contributors to learn another toolset.

For this one, I think we can use [jekyll-assets](https://github.com/envygeeks/jekyll-assets) with [autoprefixer-rails](https://github.com/envygeeks/jekyll-assets#css-auto-prefixing). That might need a restructure of SASS from assets and _sass into _assets, I think, so we'll need to see if the juice is worth the squeeze.