WIP - Add Studio support for cohorted courseware

TNL-652
@catong This is a good catch, thanks. I was thinking about plugging in the xblock type, so it would read as:

    This unit refers to...

or

    This problem refers to...

Does that work? Note that I tweaked the wording so that it works for both one invalid reference as well as for two or more. So your suggested rewording would need both singular and plural forms.

A problem with my suggestion is that I don't know if we have localized xblock type names, so we might need to use quote the name.