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/lms/doctype/lms_course/lms_course.py

@@ -210,6 +210,24 @@ class LMSCourse(Document):
             visibility="Public")
         return batches
 
+    def get_cohorts(self):
+        return find_all("Cohort", course=self.name, order_by="creation")
+
+    def is_cohort_staff(self, user_email):
+        """Returns True if the user is either a mentor or a staff for one or more active cohorts of this course.
+        """
+        q1 = {
+            "doctype": "Cohort Staff",
+            "course": self.name,
+            "email": user_email
+        }
+        q2 = {
+            "doctype": "Cohort Mentor",
+            "course": self.name,
+            "email": user_email
+        }
+        return frappe.db.exists(q1) or frappe.db.exists(q2)
+
     def get_lesson_index(self, lesson_name):
         """Returns the {chapter_index}.{lesson_index} for the lesson.
         """