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.

r? anyone?
Nope– good catch!