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's SCSS. I've replaced these with [`calc()`](https://developer.mozilla.org/en-US/docs/Web/CSS/calc), which doesn't add any functionality, but shows you which units need to be converted to `rem` units.