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/document_indexing/handlers.py

@@ -8,7 +8,7 @@ from .tasks import (
 )
 
 
-def create_default_document_index(sender, **kwargs):
+def handler_create_default_document_index(sender, **kwargs):
     DocumentType = apps.get_model(
         app_label='documents', model_name='DocumentType'
     )