Show quota muk filesets

- fix show_quota to retrieve information from fileset cache files
- allow user cache files as well for use on the tier-2 storage
- do not send mail to users on muk (see #31), this solution should be improved to actually check group membership
not really, but makes `vsc-filesystems` not portable.
@itkovian can we make a `vsc-filesystems-quota` repo to keep the dependencies and VSC-isms to a minimum in vsc-filesystems? can you estimate the impact and work needed for this? we can discuss it a bit during mondays teammeeting