Add GroupDefinition service.
Update subscriptions to not have a group name when their group is deleted.
Amend:
Rebase to HEAD of 13.2.1.
Change-Id: I817ca1912f76ccf347907ddb12e3f6fd8de7e5aa
Former-commit-id: 557de162b0 [formerly 464b46dd4f4af60dbeb917f65c18c876d712e4ad]
Former-commit-id: 15c2c55ff9