Add latest Diff timestamp. Review last activity does not represent diff activity.

Review last activity does not represent diff activity. One can not see if the
newer diff has been uploaded or just comment added. This change adds another
timestamp to the header of the review request box to show when the last version
of the diff was added.

Testing Done:
Tested manually on my development RB install.

Reviewed at https://reviews.reviewboard.org/r/4898/