Sidenav styling: Add an open/close caret icon

[cmajel]

As a cloud.gov user, I want to know what links are to pages and what are expandable categories so that its easy for me to navigate the site and I know when I'll go to a new page.

Currently, the sidenav does not indicate when a link is for an expandable category (Like Purchasing and Pricing) and when it is to an actual page ( Like Try a free sandbox space). There should be an indicator for expandable categories to make them distinct. Here's a design for what that could look like with a caret icon on the right.