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/cohorts/index.html

@@ -0,0 +1,34 @@
+{% extends "www/cohorts/base.html" %}
+{% block title %}Manage {{ course.title  }}{% endblock %}
+
+{% block page_content %}
+<div class="common-page-style">
+  <div class="container">
+    {% if cohorts %}
+      <h2>Cohorts</h2>
+      <div class="row">
+      {% for cohort in cohorts %}
+        <div class="col-md-4">
+        {{ render_cohort(course, cohort) }}
+        </div>
+      {% endfor %}
+    {% else %}
+        <h2>Permission Denied</h2>
+        <p>You don't have permission to manage this course.</p>
+    {% endif %}
+   </div>
+  </div>
+</div>
+{% endblock %}
+
+{% macro render_cohort(course, cohort) %}
+<div class="card">
+  <div class="card-body">
+    <h5 class="card-title">{{cohort.title}}</h5>
+    <h6 class="card-subtitle mb-2 text-muted">{{cohort.begin_date}} - {{cohort.end_date}}</h6>
+
+    <a href="/courses/{{course.name}}/cohorts/{{cohort.slug}}" class="card-link">Manage</a>
+  </div>
+</div>
+
+{% endmacro %}