View proposal diffs

This adds a very simple model to keep track of rendered diffs for each section of a talk proposal.

It uses the HTML diff of difflib, which only supports HTML output for side-by-side, which isn't great for the narrow layout - also it lacks great resolution of changes in long wrapped lines.

The implementation of the diffing can be swapped out, improved at a later time. The idea was to get something useful available to reviewers.