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?
Wait, did we mean to lose **normalize.css** here?
I see we still have it in the _server/templates/pages/src/404.html_ template, but we lost it in the _500.html_ (this file), and the _503.html_ template.