Complete conversion of download views to CBV views using django-downloadview.

This also removes dependency on the filetransfers library.

mayan/apps/common/tests/decorators.py

@@ -1,5 +1,5 @@
 def skip_file_descriptor_check(func):
-   def func_wrapper(item):
-       item._skip_file_descriptor_test = True
-       return func(item)
-   return func_wrapper
+    def func_wrapper(item):
+        item._skip_file_descriptor_test = True
+        return func(item)
+    return func_wrapper