New template: Before/after images

[alykat]

Can base off this graphic: https://github.com/nprapps/graphics-js/tree/main/tonga-beforeafter-20220125

Todos:

• Accessibility review (aria labels; should the toggle buttons be radio buttons instead a la the covid trackers)
• Use vanilla JS instead of D3 if possible
• Can this code be generalized so potentially multiple before/afters can be stacked in the same embed?

[shellyscheng]

Completed in pull request: https://github.com/nprapps/dailygraphics-templates/pull/36