Fix wiki login

https://sentry.thalia.nu/sentry/concrexit/issues/97/

Previous behaviour

Steps to reproduce:

1. Login to wiki

New behaviour

Steps to validate that it works:

1. Login to wiki

website/activemembers/fixtures/member_groups.json

@@ -59,6 +59,7 @@
     "model": "activemembers.committee",
     "pk": 1,
     "fields": {
+        "wiki_namespace": "testcie1:ns"
     }
 },
 {