Fix existing hex encoded json values in DB

synapse/storage/schema/delta/56/redaction_censor3_fix_update.sql.postgres

+/* 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.
+ * 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.
+ */
+
+
+-- There was a bug where we may have updated censored redactions as bytes,
+-- which can (somehow) cause json to be inserted hex encoded. This goes and
+-- undoes any such hex encoded JSON.
+UPDATE event_json SET json = convert_from(json::bytea, 'utf8')
+WHERE event_id IN (
+  SELECT event_json.event_id
+  FROM event_json
+  INNER JOIN redactions ON (event_json.event_id = redacts)
+  WHERE have_censored AND json NOT LIKE '{%'
+);