Child links in navigation not accessible via keyboard

[katknow]

Currently, the navigation fails the Accessibility Insights for Web assessment when child links are added to the navigation bar. While you can technically tab to them, it does so without opening the menu so it looks like you are tabbing to nothing. The navigation menu would need to be able to be opened and closed by keyboard to pass.

[kimisgold]

I've attempted to address this in b362181 and dc5c96b and the other previously referenced commits. The top nav should now be navigable by keyboard.

• The arrow buttons will open and close child menus when hovered in and out on as well as by click.
• On keyboard, tabbing out of a branch of the nav tree will close the child menu. Only one branch should be visible at a time.

This should be tested with screen readers on the major desktop and mobile browsers. I can cover android devices.