feat: added a page to list cohorts of a course

- added a page /courses/<course>/manage that lists the active cohorts
- only accessible to mentors and staff
- also added a "manage the course" button on course page for mentors/staff

Issue #271

school/www/courses/course.html

@@ -70,6 +70,13 @@
         <img class="ml-2" src="/assets/school/images/play.png" />
       </div>
       {% endif %}
+
+      {% if course.is_cohort_staff(frappe.session.user) %}
+        <a class="button wide-button" style="position: absolute; right: 0;"
+           href="/courses/{{course.name}}/manage">
+          Manage the course
+        </a>
+      {% endif %}
     </div>
   </div>
 </div>