refactor: Simplify root user notification and remove global email variable

- Remove global `RootUserEmail` variable
- Modify channel testing and user notification methods to use `GetRootUser()`
- Update user cache and notification service to use more consistent user base type
- Add new channel test notification type

common/constants.go

@@ -101,7 +101,7 @@ var PreConsumedQuota = 500
 
 var RetryTimes = 0
 
-var RootUserEmail = ""
+//var RootUserEmail = ""
 
 var IsMasterNode = os.Getenv("NODE_TYPE") != "slave"