Add "diff" feature to Response History

Please implement a feature that will users to compare ("diff") two responses. The user should be able to select which responses on which they would like have the diff comparison performed. A great subfeature would be to have an option to automatically update the comparison between the most recent response and the second-most recent response, or a selected fixed response from the history.

This is the "Response history" referenced in my request.


Note: This request is similar to #3552, but that request had multiple aspects. I do not feel that my request is an extension of that issue.