Add nixexprpath/nixexprinput to GET of jobset (API).

Dependencies

• [2] NP7IOJ4Q Flesh out the API a bit
• [3] 4VYSDP4I Add eager fetching of relations and enable that for jobset->jobsetinputs
• [4] YI3BZG5N Cleanup Jobset serialization
• [*] X27GNHDV * Basic job info in the database.

Change contents

• replacement in src/lib/Hydra/Schema/Jobsets.pm at line 316
  [2.218]→[2.218:242](∅→∅)
  − "emailoverride"
  [2.218]

  [2.242]
  + "emailoverride",

  + "nixexprpath",

  + "nixexprinput"