Deltas: show delta of "First Load" instead of page-specific delta

Currently, refactoring code so that some pages have a reduced size requires you to do mental math to figure out the true impact. Since what really matters is the "First Load" size of each page, I think the delta should be calculated and shown based on that.

Here's an example that requires mental math. In this case, both pages ended up 3kb smaller and *that* is the figure that I think should be highlighted.

It may be important to show the deltas on both page-specific sizes *and* `First Load` but of those, `First Load` seems more important to me.