Previous to this change, Weblate's host user home directory was stored
in `$mutable_data_dir`. This was wrong and confusing because not only
there was a lot of immutable data there, but also mutable data which is
not directly touched by Weblate itself, but by our custom code (eg.
staging and integration Git repositories).

Now we store `/var/lib/weblate` in `$weblate_home`, set Weblate's
DATA_DIR as `/var/lib/weblate/data`, and move the staging repository to
`/var/lib/weblate/repositories/staging`.