Import l10n files from mozilla-aurora

This PR fixes #3242 and supersedes #3762 and #3766. It adds a Makefile utility called `importl10n` (call: `node make importl10n`) that fetches the latest translations from mozilla-aurora. This avoids users translating strings here that are already translated by the Firefox team and makes sure that translations are always synchronized between Firefox and PDF.js.

Note that this tool keeps our `metadata.inc` files because the Firefox team does not translate those.

The PR changes the regex used in `node make firefox` a bit to allow the new languages codes from mozilla-aurora, in particular three-letter codes.