warn instructors about quiz availability when adding extra time

fixes QO-619

In Old Quizzes, when an instructor adds extra time to an
individual student's attempt/s, this extra time does not allow the
student to submit their quiz after the availability dates of the
quiz have passed. Instructors are unaware of this feature. To make
them aware of the feature, add a warning message to the pop-up
modal where instructors add time to students' attempt/s.

Test Plan:
- As an instructor, create a course and enroll a student.
- Create a quiz in Old Quizzes that has a time limit.
- Click "Moderate This Quiz." Click on the edit
  icon corresponding to the student's name. Under "Extra time on
  every attempt," you should see the following text: "Quiz
  availability dates that pass during an attempt will still
  auto-submit even if the extended time has not expired."

flag=none

Change-Id: Id068274d408c94ba946857613e6fef58392b42be
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/240401
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Product-Review: Matthew Goodwin <mattg@instructure.com>
Reviewed-by: Stephen Kacsmark <skacsmark@instructure.com>
QA-Review: Mark McDermott <mmcdermott@instructure.com>

app/views/quizzes/quizzes/moderate.html.erb

@@ -195,6 +195,11 @@
           <span class="units"><%= t('minutes', 'minutes') %></span>
         </span>
       </div>
+      <div class="field cutoff_warning">
+        <span class="cutoff_warning">
+          <p class="note"><%= t('Quiz availability dates that pass during an attempt will still auto-submit even if the extended time has not expired.')%></p>
+        </span>
+      </div>
     <% end %>
 
     <% if @quiz.one_time_results %>