adds better focus orders for services button/menu

[markconroy]

Closes #578

What does this change?

• Adds a new function in header.js so that when you click tab after last item in services menu, the focus goes back to the Services toggle button
• When you shift+tab back out of services menu, the services button does not collapse the menu any more
• Changes the aria-label to be more succinct.

How to test

• Checkout this branch
• Clear you cache
• Tab to services button in header, click it
• Tab through services links ensure that when you tab after the last one it goes back to the services button
• Tab into services menu, shift+tab back out of it, ensure that the services menu does not collapse when you get back to the services button
• Turn on screenreader, check that the aria-label for services button is "Services: jump to services"

---

Thanks to Big Blue Door for sponsoring my time to work on this.