mozilla
Add ability to download translations directly from Pontoon, without relying on VCS
Downloading translation files from Pontoon is essentially a hack, because we don't have persistent storage on our Pontoon instance and hence cannot rely on the repository clone to be accessible. So we download files using direct download links from public repository.
Pontoon however also supports private repositories (see #2545). In such case, downloading translations won't work, because public download links are not available. We should mitigate that and try to get the files from local clones. See #2546, which does the same thing for the project configuration file.
With the switch to moz.l10n and the underlying model changes, we should be able to download translations directly from Pontoon, without relying on VCS.
With a caveat that in order to get there, we need to first complete the sync refactor, and that's going to include a first rewrite of the upload and download implementations, as the old sync code they're currently relying on is going away.
