PLT-6610 sending websocket event for last channel viewed (#6787)

config/config.json

@@ -45,6 +45,7 @@
         "EnablePostSearch": true,
         "EnableUserTypingMessages": true,
         "EnableUserStatuses": true,
+        "EnablChannelViewedMessages": true,
         "ClusterLogTimeoutMilliseconds": 2000
     },
     "ElasticSearchSettings": {