Add version comparison and revert improvements

This page is a capture in the next bucket of the product backlog — a pre-sprint idea, not yet pulled into a sprint as a story.

Improvements to entity version history and comparison features.

• In the history diff dialog, add a "from version" combo box that lets users compare against any previous version (not just the immediately preceding one).
• Revert-to-version should be a server-side operation taking current version and target version parameters.