fix: allow flagging for abuse when filtering comments by user (#30346)

Abuse flagging is always supported, so allow abuse flagging even when no other
actions are available due to the lack of a thread context in the comment api.

lms/djangoapps/discussion/rest_api/permissions.py

@@ -99,8 +99,8 @@ def get_editable_fields(cc_content: Union[Thread, Comment], context: Dict) -> Se
     elif context.get("thread"):
         is_thread_closed = context["thread"]["closed"]
     else:
-        # No editable fields when outside thread context
-        return set()
+        # Flagging/un-flagging is always available.
+        return {"abuse_flagged"}
 
     # Map each field to the condition in which it's editable.
     editable_fields = {