Replace modal with USWDS modal

[maya]

https://designsystem.digital.gov/components/modal/

You'll get all of the a11y features automatically by doing this, e.g. #480

Also, the more custom styles you can remove the better (e.g. the button's using the wrong color)

cc @ryanwoldatwork

[ryanwoldatwork]

Thanks @maya!

[ryanwoldatwork]

Update: This work is in progress.

We're exploring ways to include the latest USWDS modal behavior and styles in an embedded format. The current .js packaging doesn't appear to lend itself to inclusion easily.