This patch adds a biblionumber column to course_items, adds a
relationship between course_items.biblionumber and biblio.biblionumber,
and changes course_items.itemnumber to allow null values.
Signed-off-by: Christian Stelzenmüller <christian.stelzenmueller@bsz-bw.de> Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com> Signed-off-by: Nick Clemens <nick@bywatersolutions.com> Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>