Return the thread ID properly down sync. (#14159)

A receipt's thread ID, if one exists, should be added to the
body of a receipt.

changelog.d/14159.feature

+Support for thread-specific notifications & receipts ([MSC3771](https://github.com/matrix-org/matrix-spec-proposals/pull/3771) and [MSC3773](https://github.com/matrix-org/matrix-spec-proposals/pull/3773)).

synapse/storage/databases/main/receipts.py

@@ -416,6 +416,8 @@ class ReceiptsWorkerStore(SQLBaseStore):
             # {"$foo:bar": { "read": { "@user:host": <receipt> }, .. }, .. }
             event_entry = room_event["content"].setdefault(row["event_id"], {})
             receipt_type = event_entry.setdefault(row["receipt_type"], {})
+            if row["thread_id"]:
+                receipt_type[row["user_id"]]["thread_id"] = row["thread_id"]
 
             receipt_type[row["user_id"]] = db_to_json(row["data"])