Bug 31577: Use patron category multi-select for OpacHiddenItemsExceptions system...

Bug 31577: Use patron category multi-select for OpacHiddenItemsExceptions system preference

We are now using the patron category selects on almost all system
preferences, but OpacHiddenItemsExceptions was still missing.

To test:
- Before applying the patch:
- Add patron categories to OpacHiddenItemsExceptions using |
- Add configuration to OpacHiddenItems
- Verify all works as expected in the OPAC
- Apply patch, run database update
- Verify the system preference shows the correct settings from before
- Verify feature still works as expected

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>