Improve event metadata structure

[bernhold]

We should probably take advantage of the nesting possible with YAML.

Instead of venue, venue-type, venue-url, and location, we could have

venue:
  name:
  type:
  url:
  location:

And similarly for parent-event, and maybe others.

[bernhold]

For parent events, maybe we can use parent-event-name (e.g. SC23) as a key to find the parent in the events array and get the link that way, avoiding the need to track the parent event filename too.