Thread sort key and direction for discussion api

Discussion API for thread sorting now takes the sort_by
parameter for the sort key such as by comment_count or
vote_count. Also takes the sort_direction parameter for
sorting the thread results.

lms/djangoapps/discussion_api/forms.py

@@ -54,8 +54,24 @@ class ThreadListGetForm(_PaginationForm):
     following = NullBooleanField(required=False)
     view = ChoiceField(
         choices=[(choice, choice) for choice in ["unread", "unanswered"]],
+        required=False,
+    )
+    order_by = ChoiceField(
+        choices=[(choice, choice) for choice in ["last_activity_at", "comment_count", "vote_count"]],
         required=False
     )
+    order_direction = ChoiceField(
+        choices=[(choice, choice) for choice in ["asc", "desc"]],
+        required=False
+    )
+
+    def clean_order_by(self):
+        """Return a default choice"""
+        return self.cleaned_data.get("order_by") or "last_activity_at"
+
+    def clean_order_direction(self):
+        """Return a default choice"""
+        return self.cleaned_data.get("order_direction") or "desc"
 
     def clean_course_id(self):
         """Validate course_id"""