DCC Bug 668 - Fix for issue of roles in a plan that does not have a user_id set. This has proved in several cases.
DCC Bug 668 - Fix for issue of roles in a plan that does not have a
user_id set. This has proved in several cases.

Fix for DCC bug https://github.com/DigitalCurationCentre/DMPonline-Service/issues/688

Change in the Plan model method owner_and_coowners() we check that each
role we currently get also has associated user that is not nil.

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