Import l10n files from mozilla-aurora [WIP]

This PR fixes #3242. The first commit 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.

The second commit is quite a huge one actually. It actually adds all new translations available on mozilla-aurora to our repository, thereby synchronizing both translations. It overwrites our files with the ones from mozilla-aurora, but I have kept our `metadata.inc` files because the Firefox team does not translate them. For all newly imported languages, the import command just copies the English `metadata.inc` file and changes the language code. This is done such that the extensions for all languages can be properly generated (I think the extension generator requires `metadata.inc` to be available for each available language, though I could be wrong here).

The third commit fixes a minor linting issue I discovered after submitting this PR. I don't know how to squash it with the first commit, but retain the second one, so I've not done that. If it's really necessary, I'll need to figure out a way to do this.

In the end, this PR gives PDF.js a great amount of new and improved translations and the Makefile command 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 :-)

- [x] Investigate polling Nightly channel (attemps to do this failed before and it is not a core feature for this PR).
- [x] Specify locales to download in a separate file.
- [ ] Split into two PRs: one for the Makefile utility and one for the l10n updates.
- [x] Investigate langcode errors with `node make firefox`.
- [x] Check if addon size can be reduced.
- [x] Remove placeholder `metadata.inc` file creation.