Add default push rule to ignore reactions

This adds a default push rule following the proposal in
[MSC2153](matrix-org/matrix-spec-proposals#2153).

See also element-hq/element-web#10208
See also matrix-org/matrix-js-sdk#976

synapse/push/baserules.py

@@ -248,6 +248,18 @@ def make_base_prepend_rules(kind, modified_base_rules):
         ],
         "actions": ["notify", {"set_tweak": "highlight", "value": True}],
     },
+    {
+        "rule_id": "global/override/.m.rule.reaction",
+        "conditions": [
+            {
+                "kind": "event_match",
+                "key": "type",
+                "pattern": "m.reaction",
+                "_id": "_reaction",
+            }
+        ],
+        "actions": ["dont_notify"],
+    },
 ]