Consolidate the page thumbnail and staging folder thumbnail widgets.

Update the staging folder file image API end point to work
in the same way the document page image API works.

mayan/apps/sources/serializers.py

@@ -46,11 +46,6 @@ class StagingFolderSerializer(serializers.HyperlinkedModelSerializer):
         model = StagingFolderSource
 
 
-class StagingSourceFileImageSerializer(serializers.Serializer):
-    status = serializers.CharField()
-    data = serializers.CharField()
-
-
 class WebFormSourceSerializer(serializers.Serializer):
     class Meta:
         model = WebFormSource