Jobs: add a nullable jobset_id foreign key to Jobsets.

Also, adds an explicitly named "jobs" accessor to the Jobsets Schema object, which uses the project/jobset name.

Created by  Graham Christensen  on February 5, 2020
SIDK2E7VTH36ETTT7VQWKUXIJE3GQCH4EA33TJG3S4ZTOPTKYCNAC
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