validation worker

[naggie]

sometimes failures that affect a lot of entities look like dsportal itself has issues. In order to prove failures are genuine, there should be an option to specify a worker to validate a failure.

Eg:

1. Website goes offline (timeout) checked bny local default worker
2. validation worker (from a different location, IP) runs the same check
3. If that check also fails, a notification is sent. Else, it's just a warning condition.