Add tabindex -1 to skip link target

[anthonylindsay]

##What does this change?

Adds tabindex="-1" to <main>.

This is for an IOS/keyboard navigation bug which means focus never really gets to main content when using the skip link, and keyboard-navigation IOS users effectively cannot use the skip link.

It allows focus the main content element.

How to test

This is primarily only testable with the correct hardware: an IOS device & external keyboard.

1. Load a page.
2. Navigate to the skip link
3. Hit ENTER to activate it.
4. Verify that you have been brought to the main content area (i.e. past main nav/header)
5. Hit TAB again.
6. Verify that focus is now on the next available link or focusable element within main content.