ungraded & anon discuss initiated from assignments

refs VICE-4158
flag=react_discussions_post

Test plan:
 - new selenium test passes
 - Have a course
 - Discussion/Announcements redesign feature flag ON
 - Discussion Create/Edit Page React Update flag OFF
 - Navigate to the assignments tab, select the "+" on
    the assignment group, change it to discussion
    and select more options.
 - On the discussions details page add a title, select
   "partial" or "full" under anonymous discussion,
   Remove the "graded" check, and save.
 - Check if discussion is saved succesfully

Change-Id: I149b069c4a08750015812afc7f7a7be71ccc8d67
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/348664
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Omar Soto-Fortuño <omar.soto@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Balázs Buri <balazs.buri@instructure.com>

spec/selenium/discussions/discussions_edit_page_spec.rb

@@ -523,6 +523,19 @@ describe "discussions" do
             expect_new_page_load { f(".form-actions button[type=submit]").click }
             expect(fj("span:contains('anonymous topic title')")).to be_present
           end
+
+          it "able to save anon, not graded, quick added from assignments", :ignore_js_errors do
+            get "/courses/#{course.id}/assignments"
+
+            f(".add_assignment").click
+            click_option(f('[name="submission_types"]'), "Discussion")
+            f(".create_assignment_dialog input[type=text]").send_keys("anon disc from assignment")
+            f(".more_options").click
+
+            f("input[type=radio][value=partial_anonymity]").click
+            f("input#use_for_grading").click
+            expect_new_page_load { f("button.save_and_publish").click }
+          end
         end
       end
     end

ui/features/discussion_topic_edit/backbone/views/EditView.jsx

@@ -863,6 +863,11 @@ EditView.prototype.validateBeforeSave = function (data, errors) {
       },
     ]
   }
+
+  if (Object.keys(errors).length === 0 && data.anonymous_state != null) {
+    data.set_assignment = false
+  }
+
   return errors
 }