Provider: Make 0 for survey interval mean "as many surveys as there are"

Summary:
This is what the FeedbackConfigUiController code was saying
"I will participate in web surveys whenever one is available. Surveys can of course be deferred or skipped."
but the code didn't really do that

Reviewers: vkrause

Reviewed By: vkrause

Subscribers: vkrause

Differential Revision: https://phabricator.kde.org/D23123