don't push by default for group chats.

synapse/push/baserules.py

@@ -335,6 +335,11 @@ def make_base_prepend_rules(kind, modified_base_rules):
                 "_id": "_message",
             }
         ],
+        # default to not notifying for group chats
+        # see https://github.com/vector-im/riot-web/issues/3268
+        # we can't do this on existing servers because we need to
+        # add per-user overrides to preserve their existing behaviour
+        "enabled": False,
         "actions": ["notify", {"set_tweak": "highlight", "value": False}],
     },
     # XXX: this is going to fire for events which aren't m.room.messages