1709: Header a11y improvements

[stevejalim]

This changeset addresses a11y issues with the main nav spotted by @schalkneethling (thanks!)

• remove unnecessary
• SCSS cleanup
• add quick-access nav links

Note that one recommendation from the source ticket #1709 was do drop a base-64 data-uri image from the CSS, but it looks like this was actually needed as the close trigger for the nav when in mobile-viewport mode.

How to test

• Code will be on staging by the time you read this

[stevejalim]

@schalkneethling I cross-browser tested, but Browserstack wasn't letting me get to localhost on iOS, so if you don't mind checking that, that'd be great