Issue #402: clone koha-l10n if needed
We are on the way of removing .po files from Koha src (bug 35174). But devs will still want the po to be there.
This is a first step and will certainly need to be enhanced later, but the idea is:
- Clone koha-l10n into misc/translator/po if the dir does not exist
- If the dir exists and is a git repo, we pull and checkout the correct branch