Accessibility improvements for screen readers

[natdudley]

This PR incorporates a lot of refactoring for screen reader access including:

• cleaning up HTML structure to remove unnecessary links
• removing decorative images from screen reader view
• improving menu access toggling functions
• improving menu button access

The JS is a bit messy and could probably do with a refactor but I think it works OK.

For menu, you should be able to:

• use enter to open menu and have focus move to close button
• use enter to close menu and have focus move back to menu button
• use space bar to open menu and have focus move to first link
• using a screen reader, see menu, open menu, and have focus shift into menu, and vice versa for closing menu.

[robisaacnz]

I need to hack together a local build to look at this in detail, which I'm doing now.

[natdudley]

Hmm, noticed a merge conflict now. WIll resolve locally.

[natdudley]

Fixed now.

[natdudley]

I merged the FA changes in locally and double checked so it's all g.