Make wagtailadmin migrations re-runnable

wagtail/wagtailadmin/migrations/0001_create_admin_access_permissions.py

@@ -13,20 +13,20 @@ def create_admin_access_permissions(apps, schema_editor):
     # Add a fake content type to hang the 'can access Wagtail admin' permission off.
     # The fact that this doesn't correspond to an actual defined model shouldn't matter, I hope...
     if django.VERSION >= (1, 8):
-        wagtailadmin_content_type = ContentType.objects.create(
+        wagtailadmin_content_type, created = ContentType.objects.get_or_create(
             app_label='wagtailadmin',
             model='admin'
         )
     else:
         # Django 1.7 and below require a content type name
-        wagtailadmin_content_type = ContentType.objects.create(
+        wagtailadmin_content_type, created = ContentType.objects.get_or_create(
             app_label='wagtailadmin',
             model='admin',
             name='Wagtail admin'
         )
 
     # Create admin permission
-    admin_permission = Permission.objects.create(
+    admin_permission, created = Permission.objects.get_or_create(
         content_type=wagtailadmin_content_type,
         codename='access_admin',
         name='Can access Wagtail admin'