Enable the moderator/creator to change the diff version for a whole review.

Specifically for a whole review choose:

• no diff - i.e. show annotations instead of diffs
• previous

for bonus points

• last reviewed
• date

As well as changing a review at the preview stage, it would be good to select this option in the "manage files" tab when initially selecting items for review. This could also be a sticky user pref too.