fix descending sort of teacher activity report

closes CNVS-18629
Currently, when sorting the teacher activity report by the ungraded
assignments column, for cells with multiple ungraded assignments,
it will sort in 'descending order' by the most recently submitted assignment,
as opposed to the oldest submitted assignment. This fixes that issue.

Test plan
- Create multiple ungraded assignments for several students
- Go to the teacher activity report as a teacher
- When sorting the table by ungraded assignments descending,
the rows should sort the table by the oldest submitted assignment
in each cell, as opposed to the newest.

Change-Id: I88463f2888c1aa876721957b5ab4ac234f6b5513
Reviewed-on: https://gerrit.instructure.com/48917
Tested-by: Jenkins
Reviewed-by: Spencer Olson <solson@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Josh Simpson <jsimpson@instructure.com>

Author: stderr

app/views/users/teacher_activity.html.erb

@@ -63,7 +63,7 @@
           <% end %>
           <td><%= number_to_percentage(student[:enrollment].computed_current_score, :precision => 1) %></td>
           <td><%= number_to_percentage(student[:enrollment].computed_final_score, :precision => 1) %></td>
-          <td data-number="<%= (student[:ungraded].map(&:submitted_at).compact.max || 1.day.from_now).to_i / 3600 %>">
+          <td data-number="<%= (student[:ungraded].map(&:submitted_at).compact.min || 1.day.from_now).to_i / 3600 %>">
             <% student[:ungraded].each do |submission| %>
               <%= link_to(submission.assignment.title, speed_grader_course_gradebook_path(course, :assignment_id => submission.assignment.id, :anchor => { :student_id => student[:enrollment].user_id }.to_json)) %><br/>
               <div style="padding-bottom: 5px; margin-top: -2px; padding-left: 20px; font-size: 0.8em; color: #888;">