Shutdown timer changes (#2084)

* change msg popup time from 10min to 30min

* countdown at half time

* simplify math

* move shutdown after checks

servatrice/src/servatrice.h

@@ -139,6 +139,7 @@ private:
 
     QString shutdownReason;
     int shutdownMinutes;
+    int nextShutdownMessageMinutes;
     QTimer *shutdownTimer;
     bool isFirstShutdownMessage, clientIdRequired, regServerOnly;