#47 Hidden language field

This should default to shown - but it doesn't make sense to show both "English" and "English (Update Preview)" the way I've been testing the update online, so there should be a way to hide a language from the picker (currently in the footer).

Since language metadata is stored in the language JSON file, it'll have to be part of that file.

This isn't something I want to specifically support for release. Customizing build paths is possibly worth investigating, but not directly as a function of languages. (For release, I'll just pull the update preview language out of the l10n directory.)

"Hidden languages" is another idea that could still be worth doing, but it's not needed for release, and should have a better use case before getting an implementation.