settings can be broken when -1 is entered as interval

@tomek:

I misconfigured data pulling intervals. One of them was configured to 0 and another one to -1.

When I clicked Save settings, it was accepted. But something snapped internally and now Stork doesn't let fix my "mistake". When I try to view the seetings, there's error message popping up: "Getting settings erred: Unknown Error" and the form is empty. Also, the grafana link disappeared after that.

Anyway, this is almost malicious negligence on the user side, so I'm very much ok if you want to push this to separate ticket and put it in the backlog category...