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/migrations/0014_auto_20180823_2353.py

@@ -0,0 +1,20 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.11 on 2018-08-23 23:53
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('document_indexing', '0013_auto_20170714_2133'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='indextemplatenode',
+            name='expression',
+            field=models.TextField(help_text="Enter a template to render. Use Django's default templating language (https://docs.djangoproject.com/en/1.11/ref/templates/builtins/)", verbose_name='Indexing expression'),
+        ),
+    ]