hide buttons in rubric long description dialog when not editing

fixes CNVS-15284

test plan:
- create an assignment with a rubric
- as a teacher, edit the rubric on the assignment page and click on the
  "view long description" link
- the dialog should have 'update description' and 'cancel' buttons
- now stop editing the rubric and click the link
- the buttons should be hidden
- try reloading the page and viewing non-editing first, then editing
- try viewing the page as a student, and click the linke
- the buttons should be hidden

Change-Id: I35a340da41f5e2210b0fa81881029f277fd2e0d2
Reviewed-on: https://gerrit.instructure.com/50966
Tested-by: Jenkins
Reviewed-by: Mike Nomitch <mnomitch@instructure.com>
QA-Review: Adam Stone <astone@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>

public/javascripts/edit_rubric.js

@@ -474,8 +474,8 @@ define([
       if(editing && !isLearningOutcome) {
         $rubric_long_description_dialog
           .fillFormData(data)
-          .find('.editing').show()
-          .find(".displaying").hide();
+          .find('.editing').show().end()
+          .find(".displaying").hide().end();
       } else {
         if(!isLearningOutcome) {
           // We want to prevent XSS in this dialog but users expect to have line
@@ -501,8 +501,14 @@ define([
         .data('current_criterion', $criterion)
         .dialog({
           title: I18n.t('titles.criterion_long_description', "Criterion Long Description"),
-          width: 400
-        }).fixDialogButtons().find("textarea:visible:first").focus().select();
+          width: 400,
+          buttons: []
+        });
+
+      if(editing && !isLearningOutcome) {
+        $rubric_long_description_dialog.fixDialogButtons();
+        $rubric_long_description_dialog.find("textarea:visible:first").focus().select();
+      }
     })
     .delegate(".find_rubric_link", 'click', function(event) {
       event.preventDefault();

spec/selenium/assignments_rubrics_spec.rb

@@ -263,6 +263,17 @@ describe "assignment rubrics" do
       expect(ee.first).to be_displayed
       expect(ee.last).not_to be_displayed
     end
+
+    it "shouldn't show 'update description' button in long description dialog" do
+      @assignment = @course.assignments.create(:name => 'assignment with rubric')
+      rubric_for_course
+      @rubric.associate_with(@assignment, @course, :purpose => 'grading')
+
+      get "/courses/#{@course.id}/assignments/#{@assignment.id}"
+
+      f(".criterion_description .long_description_link").click
+      expect(fj('.ui-dialog .save_button:visible')).to be_nil
+    end
   end
 
   context "assignment rubrics as an designer" do