fix(notifications): Include experiment fallback in the serializer

[mgaeta]

So my current theory is that the notification-slack-automatic feature flag is causing users to be opted into new Slack notifications, but when they're checking their settings, they only see that EMAIL is selected.

In the DB, this example user only has EMAIL notification settings.

-- 0 rows
select count(*) 
from sentry_notificationsetting
where target_id = ...
and provider = 110
;

My solution is to change the way we calculate "fallback" settings in the serializer to check the feature flag. It won't turn off notifications for the user, but it will allow them to turn them off themselves.

[manuzope]

Can we add some tests to verify the theory and to see if this change solves it?