Bug 20814: Display issue with 'Saved reports' tabs when memcached is off

Bug 20814: Display issue with 'Saved reports' tabs when memcached is off

This patch changes the configuration of the saved reports table so that
columns which were previously excluded from the table altogether are now
hidden. This prevents a JavaScript error when DataTables tries to
reference a missing column.

To test, apply the patch and go to Reports -> Use saved. You should have
at least one report category configured and assigned to reports in order
for the tabs display to be visible.

Test the table of saved reports under a variety of these circumstances:

 - memcached disabled or enabled
 - reports exist or don't exist which require update from the old
   marcxml syntax
 - cache_expiry and/or update columns are configured in Administration
   -> Columns settings to be shown or hidden

The report category tabs and the table should render correctly under all
circumstances.

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>