Create "current page" underline on header component

[brentonstrine]

The header items currently have a green underline on hover.

Update the header component to have a black underline when it is the current page by adding (class="current") to the HTML.

It should only show up under the page that the user is presently viewing, e.g. when the user is on the "Programs" page, they will see "Programs" underlined in black.