Implement font-size width rems

So right now we're using set pixel sizes which aren't necessarily great for different devices.  This PR adds a bit more flexibility.

This will change the overall font-size of MDN, and they'll look a bit bigger than before.  If you believe the font-size is too big, instead of just saying that in this PR, adjust the `font-size` on the `html` element in Dev Tools element. It's currently `100%` so adjust it to `95%`, etc.

Also, I'd like multiple eyes on this before we merge or reject.  @groovecoder @lmorchard @openjck @jezdez