Import l10n files from mozilla-aurora

This PR fixes #3242 and supersedes #3762. It adds a Makefile utility called `importl10n` (call: `node make importl10n`) that fetches the latest l10n files from mozilla-aurora. This implies that translations should now only be done at the central Mozilla repository and not here anymore. 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.

**This is just the tool to easily fetch all translations from mozilla-aurora. If this PR is accepted, I will create a follow-up PR with the actual l10n updates**

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.

The Makefile command in this PR makes sure that synchronizing the translations in the future will be as simple as running the `node make importl10n` command and committing/pushing the result.

If you have any feedback, please let me know :-)