fix(install): Better accessibility for installation flow

I'm pretty excited about this. This was actually really easy to do and noticeably improved the experience of getting through the installer with assistive technology (I tested using ChromeVox on ChromeOS by hitting Ctrl + Alt + Z to turn it on).

I also like the form markup a lot better than what we were using. I took the liberty of just adding it to the installer since themes don't really affect the installer. Let me know what you think and we can talk about adding it for 1.10. See #6356