Download custom appliance symbols from GitHub

Fix symbol cache issue. Ref https://github.com/GNS3/gns3-gui/issues/2671
Fix temporary directory for symbols was not deleted
Fix temporary appliance file was not deleted

gns3server/controller/appliance.py

@@ -41,6 +41,10 @@ class Appliance:
     def status(self):
         return self._data["status"]
 
+    @property
+    def symbol(self):
+        return self._data.get("symbol")
+
     def __json__(self):
         """
         Appliance data (a hash)