Use npm to manage A-Frame, implement bundle splitting

Switch to NPM, take 2

Fix avatar selector

Our build machine seems to have a problem pulling down GitHub dependencies
via NPM, so reverting this until I can diagnose and resolve that.

This reverts commit e02d6d40, reversing
changes made to eda4fc74.

Switch to use npm instead of yarn

Future versions have a bug which is awkward to work around; for
now, make sure we don't accidentally upgrade.

See https://github.com/donmccurdy/three-to-cannon/issues/13.