[translate.vm] give weblate permission to write to /var/lib/weblate/staging

Weblate user (or maybe another user?) should be able to run ./build-website with destdir == /var/lib/weblate/staging

Original created by @emmapeel on 12340 (Redmine)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information