Bug 35979: Check pref before inserting holds_queue background jobs

Bug 35979: Check pref before inserting holds_queue background jobs

Test plan:
Confirm that modrequest does no longer insert when pref is off :)

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Pedro Amorim <pedro.amorim@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>
(cherry picked from commit 310ba95efa7e83559134c7660f7739a6d1314524)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>