Add default push rule to ignore reactions (#5623)

This adds a default push rule following the proposal in
[MSC2153](https://github.com/matrix-org/matrix-doc/pull/2153).

See also https://github.com/vector-im/riot-web/issues/10208
See also https://github.com/matrix-org/matrix-js-sdk/pull/976

changelog.d/5623.feature

+Add default push rule to ignore reactions.

synapse/push/baserules.py

 # Copyright 2015, 2016 OpenMarket Ltd
 # Copyright 2017 New Vector Ltd
+# Copyright 2019 The Matrix.org Foundation C.I.C.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -248,6 +249,18 @@ BASE_APPEND_OVERRIDE_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"],
+    },
 ]