Add support for "staging" EDUPUB vocab

@mattgarrish  says:

> I think we have to make it so an EDUPUB file can fully validate -- including draft terms -- otherwise people will get frustrated. Getting them to build the draft terms in is too complicated.
> [however] I’m worried about dropping all the terms in as though they’re already valid.

This PR implements the following:

  * EDUPUB “specific” terms are defined as a new vocab in EdupubStagingVocab.java
  * This latter vocab is aggregated with the default SSV when checking Content Docs if the publication has the type `edupub`

@mattgarrish can you confirm this works as expected ?