Bug 16047: Making category code a required field on creating a group

Bug 16047: Making category code a required field on creating a group

This will prevent users from creating a group without a category code,
which causes a software error when you try to delete it

To test:
1) Go to Admin -> Libraries and groups
2) Create new group without category code
3) Attempt to delete the group you just created and notice software
   error
4) Apply patch
5) Create new group without category code
6) Notice you now cannot save the group without putting in a category
  code

Sponsored-by: Catalyst IT
Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Brendan Gallagher brendan@bywatersolutions.com
(cherry picked from commit 577aa86eb96160088c70008bfe85ae2c0820f547)
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>