Make show REFRESH normal priority - avoid stuck other threads

It user has too many shows. it can take a lot to refresh and other higher priorities have to wait like manual search, search, daily search, etc

From https://github.com/SiCKRAGETV/sickrage-issues/issues/589

" Change Refresh thread to Normal instead of High (avoid show refresh stuck other threads)"

sickbeard/show_queue.py

@@ -438,7 +438,7 @@ class QueueItemRefresh(ShowQueueItem):
         ShowQueueItem.__init__(self, ShowQueueActions.REFRESH, show)
 
         # do refreshes first because they're quick
-        self.priority = generic_queue.QueuePriorities.HIGH
+        self.priority = generic_queue.QueuePriorities.NORMAL
 
         # force refresh certain items
         self.force = force