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/documents/models/document_page_models.py

@@ -1,8 +1,6 @@
 from __future__ import absolute_import, unicode_literals
 
-import hashlib
 import logging
-import uuid
 
 from furl import furl
 
@@ -33,15 +31,6 @@ __all__ = ('DocumentPage', 'DocumentPageCachedImage', 'DocumentPageResult')
 logger = logging.getLogger(__name__)
 
 
-# document image cache name hash function
-def hash_function():
-    return hashlib.sha256()
-
-
-def UUID_FUNCTION(*args, **kwargs):
-    return force_text(uuid.uuid4())
-
-
 @python_2_unicode_compatible
 class DocumentPage(models.Model):
     """