Fix AdminFactory

The `AdminFactory` had a bug: `is_admin` is not a field, but
`is_superuser` is.

No tests were using the factory so this went unnoticed, but I'm about to
want to use it.

hypha/apply/users/tests/factories.py

@@ -48,7 +48,7 @@ class OAuthUserFactory(UserFactory):
 
 
 class AdminFactory(UserFactory):
-    is_admin = True
+    is_superuser = True
 
 
 class StaffFactory(OAuthUserFactory):