PLT-7508: Add settings to disable wide mention confirmation (#7416)

config/default.json

@@ -75,6 +75,7 @@
         "UserStatusAwayTimeout": 300,
         "MaxChannelsPerTeam": 2000,
         "MaxNotificationsPerChannel": 1000,
+        "EnableConfirmNotificationsToChannel": true,
         "TeammateNameDisplay": "username",
         "ExperimentalTownSquareIsReadOnly": false
     },