Feat: Skip Nav

[machikoyasuda]

closes #1388

requires #1387 and #1613

What this PR does

• Style the Skip Nav into a full-width div with yellow background
• Works like the Skip Nav here https://www.gov.uk/

To test

• When testing, do not have the developer console tool window open. The focus won't work.
• On Desktop: Go to any page, like http://localhost:8000/mst
• Click Tab on keyboard
• Once the skip nav appears, click Enter
• Click Tab again. You'll now be in the #main-content div and probably focused on a Call to Action button. You'll have skipped the Español button. You don't skip the Previous Page button though.
• Skip Navs are generally Desktop features b/c it's for users using a keyboard, but it works fine on Mobile too.
• Works better if you hide the Dev Mode bar. Hide it by adding d-none in base.html or adding .bg-danger { display: none;} to style.css

Screenshots

[machikoyasuda]

Rebased with latest dev

[thekaveman]

Interesting approach to put the <a> on the outside and the container within, makes the whole Skip bar clickable 👍