Close the new USWDS page header dropdown when clicking outside of it

[dzole0311]

Context

The new USWDS page header nav dropdowns, after being opened, persist their open state even after attempting to click outside of the dropdown (e.g visiting another nav item, or clicking outside of the dropdown).

This is a follow up from the new USWDS page header work: #1247

Priority:

Steps to reproduce the issue

1. Enable the new page header via a feature flag (ENABLE_USWDS_PAGE_HEADER)
2. Click on one of the dropdowns
3. Then click on another nav item (e.g the Exploration nav item)
4. Notice that the dropdown remains open

[AliceR]

This is how the uswds library handles the situation: https://github.com/uswds/uswds/blob/develop/packages/usa-header/src/index.js#L213 - in case that helps.
And this is the unanswered bug ticket in the react-uswds repository: trussworks/react-uswds#2592 (they actually see this as a feature, not a bug)