Use free_email in UserFactory email field

The `safe_email` Faker provider always uses `example.(net|com|org)` as
the email domain. For some reason, this causes validation errors with
dandischema.

dandiapi/api/tests/factories.py

@@ -35,7 +35,7 @@ class Meta:
         model = User
 
     username = factory.SelfAttribute('email')
-    email = factory.Faker('safe_email')
+    email = factory.Faker('free_email')
     first_name = factory.Faker('first_name')
     last_name = factory.Faker('last_name')