Fix miniconf editing - No attribute issue

Editing a miniconf, on save, the code looks in settings for
ADMIN_USERNAMES, and as it does not exist, we stack-trace and 502 to the
user.

Add the attribute to settings.py

pinaxcon/settings.py

@@ -87,6 +87,8 @@ if fail:
 
 ### Standard settings
 
+ADMIN_USERNAMES = []
+
 CACHES = {
     'default': {
         'BACKEND': 'django.core.cache.backends.dummy.DummyCache',