Update Django template reference URL from Django 1.7 to Django 1.11. Add migratons to update model definitions.

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

mayan/apps/document_indexing/models.py

@@ -159,7 +159,7 @@ class IndexTemplateNode(MPTTModel):
         help_text=_(
             'Enter a template to render. '
             'Use Django\'s default templating language '
-            '(https://docs.djangoproject.com/en/1.7/ref/templates/builtins/)'
+            '(https://docs.djangoproject.com/en/1.11/ref/templates/builtins/)'
         ),
         verbose_name=_('Indexing expression')
     )