Remove CalendarSource and external events

[marksteward]

This touches a surprising amount of stuff.

I haven't bothered removing prefixes like proposal_favourites, although that might be a nice follow-up commit. I didn't remove event_type from add_favourite etc, instead throwing a 400. I couldn't see from a glance anything else that significantly simplifies in the schedule.

[lukegb]

We can probably yeet

Website/templates/about/workshop-villages.md

Line 58 in ffa212d

Anything you submit to us for scheduling purposes can be added to the programme for people to find, and for this purpose we'll ask you for a title and short blurb for each activity/event. We're still pinning down how this will work, but it will potentially involve putting together an iCal feed of non-CFP activities in your village, which will be listed in a 'Village activities' section.

as well