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?
Generally I'm not a Huge Fan Of The camelCaseFileNames, but i'm not sure if we have a naming convention going. I did a quick `$ tree app` and I see we have:
- app/scripts/lib/app-start.js
- app/scripts/templates/change_password.mustache

But I don't see a lot of cAmElCaSe names outside of locale files. :shrug: