Fix for foreign key violation error when a department is deleted after some users have added its department_id.
Fix for foreign key violation error when a department is deleted
after some users have added its department_id.

Solution based on blog "Set Foreign keys to null"
https://til.hashrocket.com/posts/jqilnp3d0h--set-foreign-keys-to-null

Fix for issue #2088

DigitalCurationCentre/roadmap,CDLUC3/roadmap,DigitalCurationCentre/roadmap,DMPRoadmap/roadmap,CDLUC3/dmptool,DigitalCurationCentre/roadmap,DMPRoadmap/roadmap,CDLUC3/roadmap,CDLUC3/dmptool,CDLUC3/dmptool,CDLUC3/dmptool,CDLUC3/roadmap,CDLUC3/roadmap,DMPRoadmap/roadmap