Update talks.html for autumn 2020

[AaronC81]

Moves the last set of talks to Past talks, and adds the new talks for Autumn Term 2020.

Review of the content to check I've not got anything wrong is much appreciated but please don't merge yet, I haven't tested building the site!

[sersorrel]

this brings up the issue of year numbering – for the sake of example, what should we call the three terms of last academic year?

"autumn 2019", "spring 2020", "summer 2020" is what's implied by this PR; i think that this is confusing and that we should use either "autumn/spring/summer 2020" or "autumn/spring/summer 2019/20" instead, i'm but open to opinions from others (or maybe we already use the "19/20/20" numbering elsewhere and i haven't noticed).

[AaronC81]

That's very true - my personal preference would be "2019/20" since that's completely unambiguous but also consistent between each term for an academic year

[AaronC81]

This renders fine after a few tiny tweaks, so I'll merge it in

[AaronC81]

...whoops