Code style cleanups

Add keyword arguments. Sort arguments and models.
Move literals to their own module. Prepend handler_ to
signal handlers.

Signed-off-by: Roberto Rosario <roberto.rosario.gonzalez@gmail.com>

mayan/apps/ocr/runtime.py

@@ -12,7 +12,7 @@ from django.utils.module_loading import import_string
 from .settings import setting_ocr_backend, setting_ocr_backend_arguments
 
 ocr_backend = import_string(
-    setting_ocr_backend.value
+    dotted_path=setting_ocr_backend.value
 )(
     **yaml.load(
         stream=setting_ocr_backend_arguments.value or '{}', Loader=SafeLoader