Added Custom migration for user model

A custom migration needs to be created in Django 1.10 for the user model inherided from AbstractUser

course_discovery/apps/core/migrations/0003_auto_20170522_0856.py

+# -*- coding: utf-8 -*-
+# Generated by Django 1.10.7 on 2017-05-22 08:56
+from __future__ import unicode_literals
+
+import django.contrib.auth.validators
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('core', '0002_partner_studio_url'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='user',
+            name='username',
+            field=models.CharField(error_messages={'unique': 'A user with that username already exists.'}, help_text='Required. 150 characters or fewer. Letters, digits and @/./+/-/_ only.', max_length=150, unique=True, validators=[django.contrib.auth.validators.UnicodeUsernameValidator()], verbose_name='username'),
+        ),
+    ]