Default to no cutoff by using default value for string (null)

My last change didn't handle migrations of existing jobs very well.  Because the default value of `int` in Java is 0, `cutoffHours` would default to 0 for existing configurations.  This means that, by default, all jobs would be excluded :(

I've fixed this by representing `cutoffHours` as a String internally and converting it to an `int` only when passing it to the build choosers.  Since the default value of `String` is `null`, so `parseCutoffHours()` will do the right thing and convert that to `-1` (no cutoff) when calling the build choosers.

This has the side-effect of making the UI more intuitive: instead of entering "-1" to indicate no cutoff, users can simply leave the field blank.