Fixes apache#8187: pulsar-dashboard django migration fix (apache#8188)

Fixes apache#8187

### Motivation

The length of the consumerName in pulsar-dashboard's django model was increased to 256 in apache#4716 however the change was not propagated by the django migrations system. The existing migrations result in a size of 64:
https://github.com/apache/pulsar/blob/e65875b99e3985630e3147c2579e91da4c3973bf/dashboard/django/stats/migrations/0001_initial.py#L56

This pull request fixes this. `./manage.py makemigrations` was used to generate this migration.

### Modifications

An additional django migration file is included to update the db to match the model.

dashboard/django/stats/migrations/0003_updates.py

@@ -0,0 +1,28 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.29 on 2020-10-01 14:07
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('stats', '0002_support_deleted_objects'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='consumer',
+            name='consumerName',
+            field=models.CharField(max_length=256, null=True),
+        ),
+        migrations.AlterUniqueTogether(
+            name='subscription',
+            unique_together=set([('name', 'topic', 'timestamp')]),
+        ),
+        migrations.AlterIndexTogether(
+            name='subscription',
+            index_together=set([]),
+        ),
+    ]