add ExperimentalHideTownSquareinLHS in config setting (#8751)

config/default.json

@@ -91,6 +91,7 @@
         "EnableConfirmNotificationsToChannel": true,
         "TeammateNameDisplay": "username",
         "ExperimentalEnableAutomaticReplies": false,
+        "ExperimentalHideTownSquareinLHS": false,
         "ExperimentalTownSquareIsReadOnly": false,
         "ExperimentalPrimaryTeam": ""
     },