Attribution box should be keyboard-accessible

[ggeisler]

A user navigating the viewer with a keyboard cannot gain focus of the Attribution box, and thus cannot close it without using a mouse.

[snydman]

Should there also be mouse and keyboard controls to toggle this on and off later in the same session for the same object?

[edsilv]

If you add tabindex="0" to the attribution close button it should be accessible in the tab order.

[edsilv]

@snydman It's possible that we could minimise instead of hide the attribution div entirely.

[mejackreed]

Resolved upstream