Somehow expose diffs when submissions or reviews change (minor feature wishlist)

[PeterSewell]

As an author, especially close to the deadline when several co-authors might be changing the form data on a submission, it would be handy to see an actual diff, rather than just a list of the changed fields.

As a reviewer (maybe also as a a chair or as an author), it would be handy to see a diff when reviewers change their review.

(p.s. hotcrp is still great - thanks for it! :-) )