Replace agency select navigation for accessibility

[levinmr]

Summary

The agency tag is used to navigate to different URLs in the site on both the main page and the data page.

• This was flagged as an accessibility concern by pa11y as being within a form without a submit button.
• There's also another concern that is not flagged by any of our accessibility tools, but I found during research of how to address this issue.
• The select dropdown navigates to other URLs when the "onchange" handler for the select is triggered. This may be an issue for screen readers because it is not obvious that changing the selected option will cause you to navigate to a new page.

Solution

This pull request changes the navbar of the main site to use a USWDS navbar with links in a dropdown menu. This is not likely to be merged unless a requirement comes in to change the select onchange behavior