Introduce the separation between book copy and book issue

[NSeydoux]

Right now, the data model doesn't make a distinction between a book copy (the physical book on your shelf) vs a book issue (the immaterial notion of a certain book, like "Blacksad T1: Quelquepart entre les ombres". A book issue is by essence unique, and it can have multiple copies. CBO provides terms to make this distinction properly.

This enables to separate what is user-centric data (what books do I own, where are they right now) and community-shared data (who wrote that book, what are the different volumes of that series...). The former will refer to the former.