ClaimDifferenceVisualizer code cleanup

I wanted to highlight the actual changes in the snak details but
gave up for now and added a TODO instead.

The CSS change removes a misplaced padding from the diff view.

Change-Id: Ib5c20d2c86b7b8b9cf51d860aa5577e84f92916a
Merge "ClaimDifferenceVisualizer code cleanup"