Bug 16055: Do not allow basketgroup deletion unless empty

Bug 16055: Do not allow basketgroup deletion unless empty

If a basketgroup has basket attached, it should not be deletable.
This patch just removes the Delete button from the interface if it
cannot be deleted.
When editing a basketgroup, the "Delete basket group" button is no
longer displayed.

Test plan:
1/ Create a basketgroup
2/ Attach 1+ baskets to this basketgroup
3/ Confirm you are not able to delete it
4/ Remove the baskets from this basketgroup
5/ Confirm you are able to delete it

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Brendan Gallagher <bredan@bywatersolutions.com>
(cherry picked from commit be33e1b46537d85b3d49abc076348916dd50004b)
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>