Bug 31832: Add reference for EnableItemGroups to EnableItemGroupHolds system preference

Bug 31832: Add reference for EnableItemGroups to EnableItemGroupHolds system preference

This patch adds a note and a link to the EnableItemGroups system
preference from the description of EnableItemGroupHolds.

To test:
1. Apply patch
2. Go to Administration > System preference
3. Search for EnableItemGroup
4. Read the description of EnableItemGroupHolds, make sure the grammar
   and spelling are OK.
5. Click the link, it should bring you to EnableItemGroups

Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>