Import l10n files from mozilla-aurora [WIP]

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.

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.

- [x] Move the download logic to `external/importL10n` (say `locales.js`) in a function `downloadLanguageFiles(lang, dest)`.
- [ ] Squash the last three commits.
- [x] Update the wiki with the message from `CONTRIBUTING.md`.
- [x] Review comment on text in `CONTRIBUTING.md`