Add a column to the submissions table to show when a submission was edited

I love that people can easily edit their submissions! It would be nice to have a column in the submissions table that would indicate when the submission was last edited. Since we will be using JotForm for proposal submissions, this will be helpful if someone asks why something was changed. We can at least say..."It was changed on March 18 at 1:00 p.m.)

It won't tell us what was changed, but at least we will know when, and that may help jog memory.