Feature/ax updates

[sarah-storm]

Various AX fixes, bundled into one PR as most are small.

#55 - Expandable navigation - focus position

• Updated AC to match functionality.
• Maybe a wee bit of design input needed to think of a better way to show the 'active' link so that it's not confused with focus. Or maybe we don't need to show that at all.

#56 - Tabbable main element is confusing on example page

• Removed tab index and added some sample content with link instead.

#57 - Full screen navigation > landscape mobile overflow hidden

• CSS overflow fix added

#58 - Use search landmark on search patterns

• role="search" added to search forms

#61 - Modal should be contained in a landmark

• Added a role="region" to both and updated ACs. I did the same for both as the form already had a role="search" applied from Use search landmark on search patterns #58 and didn't want to nest them?

#54 - Expandable navigation - NVDA

• This was the biggest update. Gov.uk are achiving this by toggling the aria-label when the button is used. Prior to this, our button's aria label was announced when landing on the button, but never -re-read as the button was used. NVDA will now read out an updated label when the button is pressed, but I'm not sure what thoughts are on this? Whether it might cause issues elsewhere?

[mjbp]

Merged with a couple of tweaks.

I think we should give the govuk navigation toggle button labelling pattern a go for now.