prevent users from ever changing the category of a topic in a support category

spec/controllers/topics_controller_spec.rb

@@ -898,6 +898,18 @@ describe TopicsController do
           end
         end
 
+        context 'when topic is in support category' do
+          let(:another_category) { Fabricate(:category) }
+
+          it "cannot change the category of a topic that is in a support category" do
+            @topic.category = Fabricate(:category, is_support: true)
+            @topic.save!
+            xhr :put, :update, topic_id: @topic.id, slug: @topic.title, category_id: another_category.id
+            expect(response).not_to be_success
+          end
+
+        end
+
         context "allow_uncategorized_topics is false" do
           before do
             SiteSetting.stubs(:allow_uncategorized_topics).returns(false)