Move artifacts to event.yml

[bernhold]

This will give us one less data file to keep track of. It also consolidates the information we'll want to share with the i-p.o web site, etc.

Most places where the artifacts appear, I think they are assigned to a my-artifacts variable to extract them from he whole _data data structure and make them more conveniently accessible. So we ought to be able to pretty easily look for them in as separate file or within events and set my-artifacts accordingly and everything else should work fine. In theory.

[bernhold]

The layouts and utilities pages are now setup to use artifacts from event.yml if found, then the separate artifacts.yml.