Middleware: Modernize middleware classes

Make the custom middleware provided by Mayan to use the
MiddlewareMixin provide by Django. This make the middleware
classes behave like classes or callables. This change ensures
compatibility with Django 2.x.

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

mayan/apps/common/middleware/error_logging.py

@@ -4,11 +4,12 @@ import logging
 
 from django.core.exceptions import PermissionDenied
 from django.http import Http404
+from django.utils.deprecation import MiddlewareMixin
 
 logger = logging.getLogger(__name__)
 
 
-class ErrorLoggingMiddleware(object):
+class ErrorLoggingMiddleware(MiddlewareMixin):
     def process_exception(self, request, exception):
         if not isinstance(exception, (PermissionDenied, Http404)):
             # Don't log non critical exceptions