Put l10n files in a different repository

[bugzilla-to-github]

This issue was created automatically by a script.

Bug 1504698

Bug Reporter: @adngdb Depends on: Bug 1486500

I dislike mixing localization commits with code commits, so I would like Pontoon to have its localization files outside the main repo. We have a mozilla-l10n organization on github for this specific use case: https://github.com/mozilla-l10n/

This bug covers:

• setting up a repo for pontoon-l10n
• adding code to deploy l10n files on stage and production
• adding code to deploy l10n files on development
• figuring out where to put the en-US file with original strings