Add migration script

To port existing rule actions & enable entries to the new name

synapse/storage/schema/delta/34/push_display_name_rename.sql

+/* Copyright 2016 OpenMarket Ltd
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+UPDATE push_rules SET rule_id = 'global/override/.m.rule.contains_display_name' WHERE rule_id = 'global/underride/.m.rule.contains_display_name';
+UPDATE push_rules_enable SET rule_id = 'global/override/.m.rule.contains_display_name' WHERE rule_id = 'global/underride/.m.rule.contains_display_name';