validate group_category_id in discussion topics

refs CNVS-16708

test plan:
 * post a new discussion topic for a group
 * it should work

Change-Id: I0e98b1000383297f21273fe66f2175af9a69d009
Reviewed-on: https://gerrit.instructure.com/43874
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Braden Anderson <braden@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>