Update WCAG implementation to use aria-modal over aria-hidden

[danieliser]

Per the latest spec we can drop the adding of aria-hidden to all elements outside the popup and use the much simpler option of just adding aria-modal to the popup when it is opened.

• [ ] On popup open add the aria-modal="true" to the popup container.
• [ ] On popup close set it to false

https://www.w3.org/TR/wai-aria-practices/examples/dialog-modal/dialog.html