Fix foundation

This pull request resolves 3 problems:

1. We were using `<script>` tags for CSS (instead of `<link>`).
2. The original CDN [wasn't actually serving the right file](//cdnjs.cloudflare.com/ajax/libs/foundation/5.4.1/css/foundation.min.css), which is a bug on their end.
3. We were using `rem-calc()`, which requires Foundation as SCSS. I've replaced this with [calc()](https://developer.mozilla.org/en-US/docs/Web/CSS/calc) which doesn't actually add any functionality here, but shows you which things need to be replaced with `rem` units.