Resolves #503 hide empty component versions (second implementation)

This is a small compact implementation of hiding components and component versions that have no page content as of the content classification pipeline stage. If a page is marked page-publishable: false as proposed in #499 (closed) this code will not be aware of that flag and assume the component-version does have that page as content.

I've verified this works against an example project https://gitlab.com/djencks/simple-examples/tree/issue-503-hidden-versions with all the possibilities I can think of. If there's some feedback that the approach in this MR is reasonable I'll write some unit tests.