Handle keyboard navigation

[dev-nicolaos]

@DoctorBracewell this PR makes a couple changes to allow the site to be navigated with a keyboard:

• changes non-link click targets to HTML <button>s
  • allows them to be focused and provides built-in space and enter key handlers
• manages focus for the song selection modal
  • sends focus to first focus-able element (first song) on open
  • makes the rest of the app inert while open to trap focus within the modal
  • sends focus back to modal trigger when modal is closed