Whoops, make our new UserManager extend the Django core one, since it turns out it's absolutely mandatory

users/migrations/0012_auto_20180129_1614.py

@@ -0,0 +1,22 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.9 on 2018-01-29 05:14
+from __future__ import unicode_literals
+
+from django.db import migrations
+import users.models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('users', '0011_auto_20180124_1311'),
+    ]
+
+    operations = [
+        migrations.AlterModelManagers(
+            name='user',
+            managers=[
+                ('objects', users.models.UserManager()),
+            ],
+        ),
+    ]