Make HTML more semantic for screen readers etc

This converts the HTML to be a list with semantic-ish tag names (less div soup) / aria roles.
I'm hoping that this means that screen readers will produce nicer output but I haven't actually had a chance to test yet. (Will do soon!)
As ever, the code can be tested on [fpp.me.uk](https://fpp.me.uk/), so if anyone's got a phone w/ accessibility tings (looking at you @benjojo) then I'd appreciate some comments.