Refactor UI Shell in Accordance with W3C Authoring Practices Guidelines

[dakahn]

closes #3572
closes #3576
closes #3590
closes #3661
closes #3591
closes #3584
closes #3583
closes #3578
closes #3580

UI Shell has serious accessibility issues necessitating a larger refactor effort getting it more in line with the Authoring Practices menu navigation example, and the their Menu Button example,

That said I do not think we should implement a role="menu" style keyboard navigation pattern (which is counter to some of the APG's guidance) and instead keep our simpler tab based implementation (ref: Heydon Pickering, Adrian Roselli)

It'd be cool if we could take this opportunity to implement React Hooks based updates (as seen in @vpicone work on our website). We should also consider UI Shell's functionality and usability when it comes to mobile since that's a focus moving forward.