Backport notification delays from master

This is a basic backport of `initial_failure_delay` and `repeat_failure_delay` from master into maint/1.x. Only the property defined on checks was backported; setting the property in the global configuration or on an entity is not currently supported.

I needed this for my environment, and I hope someone else finds it useful!
I think it makes sense to allow 0 to mean no delay at all - alert on the first failure. @ali-graham any reason not allow 0?