Explicitly nullify web_conference reference

When WebConference#destroy is called the association with the
calendar_event child is set to :nullify. Rails is supposed to nullify
that relationship as part of the destroy callbacks. I wasn't able to
recreate the issue happening in practice, we even have tests that are
checking it already.

The logs in Splunk are showing that it is still
happening occasionally so the fix here is to nullify the association
manually before the record is deleted. It's calling the same
method used in the callbacks for the destroy call.

fixes LS-3145
flag=none

test plan:
- Create a web conference
- Create a calendar event and attach the web conference
- Delete the web conference
- Verify the deletion was successful and that the calendar event is still
there but has no relationship to a web conference

Change-Id: Id3d3c6205da3367d2ac5d74ad8c454c890fab44f
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/292815
Product-Review: Eric Saupe <eric.saupe@instructure.com>
Reviewed-by: Ed Schiebel <eschiebel@instructure.com>
QA-Review: Ed Schiebel <eschiebel@instructure.com>
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>

app/controllers/conferences_controller.rb

@@ -457,6 +457,7 @@ class ConferencesController < ApplicationController
     if authorized_action(@conference, @current_user, :delete)
       @conference.transaction do
         @conference.web_conference_participants.scope.delete_all
+        @conference.calendar_event&.update_columns(web_conference_id: nil) # explicitly nullify the calendar_event
         @conference.destroy
       end
       respond_to do |format|

spec/controllers/conferences_controller_spec.rb

@@ -336,6 +336,20 @@ describe ConferencesController do
     end
   end
 
+  describe "DELETE 'destroy'" do
+    it "removes participants and the calendar event association" do
+      user_session(@teacher)
+      @conference = @course.web_conferences.create!(conference_type: "Wimba", duration: 60, user: @teacher)
+      @conference.users << @student
+      @conference.calendar_event = calendar_event_model
+      @conference.save!
+      delete "destroy", params: { course_id: @course.id, id: @conference.id }
+      expect(response).to be_redirect
+      expect(WebConference.exists?(@conference.id)).to eq(false)
+      expect(@event.reload.web_conference_id).to be_nil
+    end
+  end
+
   context "LTI conferences" do
     before(:once) do
       Account.site_admin.enable_feature! :conference_selection_lti_placement