Bug 7935: Introduce sys pref to control 'browse results' in OPAC

Bug 7935: Introduce sys pref to control 'browse results' in OPAC

Patch introduces a new system preference "OpacBrowseResults" to control
the feature for browsing and paging through results shown on top of the
left menu on detail pages in OPAC.

Preference is activated by default and can be deactivated using the
system preference.

To test:
 - Check database update works correctly
 - Check that browsing and paging still works with after database update
 - Deactivate the feature by setting 'OpacBrowseResults' to 'Disable'
 - Check the feature does no longer show up in OPAC
 - Check that a new installation also has the system preference with correct default

Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>