Disk space 12126

This aims to address http://trac.openmicroscopy.org.uk/ome/ticket/12126 and fix a couple of other issues with disk usage stats:

 - Bug: Big Images disk usage was calculated via pixels counting and gave incorrect results
 - RFE: Show usage by group, E.g. https://www.openmicroscopy.org/community/viewtopic.php?f=4&t=7485
 - RFE: Show usage by individual users / groups

To test, log in as Admin, go to "Statistics" page in webadmin.
 - Should see drivespace pie-chart of Users.
 - 'Progress bar' shows percentage of total space used. (NB: we don't show 'free space' in the pie chart now, since this made it tricky to see users if their usage was small).
 - Table shows values for each user.
 - Click on 'Groups' above the chart...
 - Now you should see a chart of data usage for each group.
 - Clicking on a segment of the chart (group) will show data usage for that group, split by user.
 - Clicking on a segment of the 'Users' chart will show data usage for that user, split by group.

If possible, try to validate that the disk usage stats are somewhat accurate, by checking the space used in the binary repository itself. NB - bug in this is fixed by https://github.com/openmicroscopy/openmicroscopy/pull/2261