WIP Issue 503 hide empty component versions

Hiding component versions and whole components that have no page content from the component explorer and page version selector is pretty easy. This implementation is biased towards the idea that we might want to hide component versions that do have page content; probably it could be simplified slightly.

Dan objected to this implementation on architectural grounds. For now, I suggest keeping it around to compare with a more compact version !394 as this one does recognize the proposed page-publishable: attribute #499 (closed).