Add skip to main content link

[dsamojlenko]

• Adds a "Skip to main content" hidden link at the top of the page (becomes visible on focus)
• Style/markup mostly stolen from Gov.UK design system version of same.
• See also: https://webaim.org/techniques/skipnav/

[timarney]

If you rebase this it should pass the tests.

[timarney]

Let's merge this - wondering why the skip link doesn't have a focus outline but we can chat with @JuliannaR next week.

[dsamojlenko]

I'm assuming the focus outline is just not visible because I set the background to the same yellow... so we can adjust. Like I mentioned in comment above, mostly just stole styles from Gov.UK for initial implementation ... we can make it our own in collaboration with Julianna for sure.