Add migration that ensures there is always a system user available

This makes migrations more robust and allows them to reference such a
system user, which becomes in handy in data migrations.

CHANGELOG.md

@@ -6,6 +6,9 @@
 
 - Python 3 is now required for the back-end. We recommend the use of Python 3.6.
 
+- A database migration ensures there will be always at least one system user
+  available. If there isn't an inactive system user is created. Such a user is
+  used whenever the CATMAID back-end needs to perform tasks on its own.
 
 ### Features and enhancements
 

django/applications/catmaid/migrations/0050_ensure_system_user.py

@@ -0,0 +1,38 @@
+# -*- coding: utf-8 -*-
+
+from django.conf import settings
+from django.core.exceptions import ImproperlyConfigured
+from django.db import migrations, models
+
+from catmaid.apps import get_system_user
+
+
+def forward(apps, schema_editor):
+    User = apps.get_model('auth', 'User')
+    try:
+        system_user = get_system_user(User)
+    except ImproperlyConfigured:
+        system_user_name = 'catmaid-system-user'
+        i = 0
+        # Find a username that isn't taken yet and create an inactive super
+        # user. It can't be used to log in.
+        while True:
+            user_name = '{}{}'.format(system_user_name, (str(i)) if i > 0 else '')
+            if User.objects.filter(username=user_name).count() == 0:
+                User.objects.create(username=user_name, is_active=False,
+                        is_staff=False, is_superuser=True)
+                break
+
+
+class Migration(migrations.Migration):
+    """This migration makes sure there is at least one inactive system user in
+    the database. This user is needed in future data migrations."""
+
+    dependencies = [
+        migrations.swappable_dependency(settings.AUTH_USER_MODEL),
+        ('catmaid', '0049_volume_tin_representation'),
+    ]
+
+    operations = [
+        migrations.RunPython(forward, migrations.RunPython.noop),
+    ]