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

[danielbeardsley]

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.