Bug 33672: Don't show item group holds on OPAC if not enabled

Bug 33672: Don't show item group holds on OPAC if not enabled

This patch adds a check of the syspref 'EnbaleItemGroupHolds' to the
opac request form

To test:
1 - Enable 'EnableItemGroups' system preference
2 - On staff client, view a record details
3 - Go to 'Item groups' tab, add a enw group
4 - On holdings tab, select one or more items and add to group
5 - View record in OPAC
6 - PLace a hold
7 - Note group option is visible
8 - Apply patch
9 - Refresh, no group option
10 - Enable 'EnableItemGroupHolds' system preference
11 - Refresh opac page, note group option is visible

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>