Do not allow adding Crowd application permissions for the same groups multiple times

Currently, when trying to add groups, the user will be offered all groups which can legally be added to the crowd application. This includes groups which have already been added. We should remove all groups already in the crowd groups when offering options for addition.

Environment:
Inside the Crowd Administration's "crowd" application, on a default installation,

Steps:

• Go to the "Groups" tab
• Click the "Add Groups" checkbox

Result:
The same group that is already added is offered for (re-)addition

Expected Result:
No groups are shown, because all available groups have already been added