Header: Add active class to menu items so those items show as active when on that page

[mshaver]

Description

The WP menu system should allow us to add an active state to menu items so the dropdown menus in the Base Build show as active when the user is on that page. Currently this isn't the case, so users don't know what page they are on when they open the menu again.

An example of this is on https://live-nsliy.pantheonsite.io/about/impact/ where the Impact menu item should have an active state.