Use main tag for main content

[ds125v]

We should override this renderer:

https://github.com/moodle/moodle/blob/master/lib/outputrenderers.php#L554-L562

So that instead of outputting this:

<div role=main>stuff</div>

It outputs this:

<main role=main>stuff</main>

There's a tiny chance this might break some overspecified javascript somewhere, but I think it's unlilkey.

I would like to also move the id=maincontent to this tag, rather than a span tag within it, but I have absolutely no idea how the span tag gets from the function it's written out to, to the place on the page, seems like some dark magic going on to do with containers? (And why didn't they add the role=main stuff here too?)

https://github.com/moodle/moodle/blob/master/lib/outputrenderers.php#L851

It's still a renderer so we can override it but i don't like changing things I don't understand.