3586: Set job priority column default to '0', add NOT NULL constraint.
3586: Set priority to "0" if nil.  Fix test fixtures.
3586: Migrate "priority" column so it is an integer instead of as string.
Merge branch 'master' into 3586-job-priority

Conflicts:
	services/api/db/structure.sql
Merge branch 'master' into 3586-job-priority closes #3586
Merge branch '3586-job-priority' closes #3586