Add helper command to extract strings to an external repo

[TheoChevalier]

This is related to MozillaFoundation/mofo-devops#735

Once the new workflow is live, translation files will live in an external repo. The string extraction will still happen in this repo, so this PR adds a script facilitating the string extraction.

It knows where to find the external repo via an env var. Mine looks like LOCAL_PATH_TO_L10N_REPO=/Users/theochevalier/Documents/GitHub/fomo-l10n/

What the script does:

• Import the latest translation files from your local copy of the fomo-l10n repo to the right places in your local foundation.mozilla.org repo
• Run makemessages to update all .po files
• Export the updated .po files to the fomo-l10n repo

Then if everything looks right, I can easily commit & push the updates to fomo-l10n

[patjouk]

LGTM, just a few code styling errors to fix: https://travis-ci.org/mozilla/foundation.mozilla.org/jobs/650100919#L252-L255
It's not a blocker, but adding a small paragraph with a link to the l1on repo and how to import/export translations would be useful for devs. It could be a new file in the docs directory or in the main readme.

[TheoChevalier]

Thanks! Fixed the tests, and added some doc in the readme

[patjouk]

All good! The review app is failing for now because compilemessages is compiling all the .po files from the virtualenv oO, but that's no need to worry/investigate since we're removing that step.
Let's merge the code that use the new translations archive, and then this PR.