Allow book content to use existing content

[mkpelletier]

Enhancement

It would be incredibly useful if the interactive book (and other combined content types) allowed for the inclusion of existing content. This would allow course designers to modularise pieces of course content and reuse it elsewhere. For example, I want to be able to create question sets that I use in a larger quiz. It would be great if I could simply reuse the content later on in a course exam for summative evaluations.