feat: added cohort and subgroup pages

Issue #271

school/www/cohorts/cohort.html

@@ -0,0 +1,25 @@
+{% extends "www/cohorts/base.html" %}
+{% block title %}Manage {{ course.title  }}{% endblock %}
+
+{% block page_content %}
+<h2>{{cohort.title}} <span class="badge badge-secondary">Cohort</span></h2>
+
+<h5>Subgroups</h5>
+<ul class="list-group">
+  {% for sg in cohort.get_subgroups(include_counts=True) %}
+    <li class="list-group-item">
+      <div>
+          <a class="subgroup-title" href="/courses/{{course.name}}/subgroups/{{cohort.slug}}/{{sg.slug}}">{{sg.title}}</a>
+      </div>
+      <div style="font-size: 0.8em;">
+        {{sg.num_mentors}} Mentors
+        |
+        {{sg.num_students}} Students
+        |
+        {{sg.num_join_requests}} Join Requests
+      </div>
+    </li>
+  {% endfor %}
+</ul>
+{% endblock %}
+