Sidenav styling: Add pill styling for active page

[cmajel]

On the sidenav, we currently what page and categories are active through color and an active pill (blue link) on the top parent:

However, this active pill would be more useful in signaling the active page, rather than the active parent category. To do this, we would need to show the pill only on the most nested .usa-current item. Here is the original design that we weren't able to fully implement: