disable submitting form with closed status but no closed_on date

This PR hides the closed_at date unless status is changed to closed, and requires the closed_at date once that is the status (but doesn't require it when hidden)

@nevern02 This could be merged as is, it works, but I think I'm missing something on how to do the form validation correctly, I have a hack comment on the thing I dislike and a couple commented outlines nearby that I tried and didn't work.