Update UI to highlight information that has changed since prior month

[lauriemerrell]

Spinning out from #256 (comment).

We assume that agencies are most interested in information in their report that is changing (for example, a new validation error that wasn't there last month, or a guideline check that was failing but is now passing). We want to consider ways that changed information could be highlighted in the UI, to draw attention to the things that the agency cares most about.