Group owners can see group disk usage

This allows group owners to see a summary of disk usage within groups they own.

To test:
 - Log in as a user who is owner of one or more groups
 - Goto top-right menu: User settings (Hmm - maybe this should be re-named?)
 - Click on Statistics Tab
 - Personal data is loaded initially. Links are provided to load disk usage for groups you own.
 - Try loading data from your groups (display of data in chart, table etc. has not changed).