Refactor Iframe Routing Logic + Add Psuedo-404 Support

[sghoweri]

Summary of changes:

• Consolidates the iFrame routing logic to be less complex. Not a complete re-write here but definitely a bit less code than where we were previously at
• Patches up some annoying browser history issues when going forward / backward
• Adds "psuedo-404" support so trying to navigate to a Pattern Lab URL for a pattern that doesn't exist (ex. http://localhost:3000/?p=atoms-foobar) or clicking on a broken link within Pattern Lab should now take you to a simple 404 error message + link back to your main default page (see GIF above^) CC @thethomic

[bmuenzenmeyer]

works great @sghoweri !

[sghoweri]

Thanks @bmuenzenmeyer! 👍🏼