fix(l10n): disable fira sans for unsupported locales

This PR does a couple of things:

* Generates a new "main" CSS file that uses `message-box` as the font for everything. Locales that are not supported use this CSS instead of `main.css` and the custom fonts.
* Translates *all* of the server page templates during the build, not just the 5xx error pages.

Fixes #1161.
Fixes #1086.

r? anyone?
We're using mixins instead of a variable here because we have to use different properties. We either use the `font` property when setting the system font or `font-family` when setting Fira. We can't simply use `font` for both cases because we'd have to include font size for the Fira case.