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/utils.py

@@ -0,0 +1,17 @@
+import frappe
+
+def get_course(course_name=None):
+    course_name = course_name or frappe.form_dict["course"]
+    return course_name and get_doc("LMS Course", course_name)
+
+def get_doc(doctype, name):
+    try:
+        return frappe.get_doc(doctype, name)
+    except frappe.exceptions.DoesNotExistError:
+        return
+
+def add_nav(context, title, href):
+    """Adds a breadcrumb to the navigation.
+    """
+    nav = context.setdefault("nav", [])
+    nav.append({"title": title, "href": href})