hrjob: Add "Initial Join Date" and "Final Termination Date"

1. Add a custom-data group called "Job Summary" (single-value)
2. Include two view-only date fields called "Initial Join Date" and "Final Termination Date"
3. Add a hook (hook_civicrm_post) or trigger (hook_civicrm_triggerInfo) which runs after any INSERT/UPDATE/DELETE to an HRJob. The hook or trigger should:

• Find min(period_start_date) for the contact and copy it to the "Initial Join Date"
• Find min(period_end_date) for the contact and copy it to the "Final Termination Date"
  4. Add a unit-test to ensure that the values are maintained when CRUDing/reassigning jobs.
  5. Update the HRDetail report-template to support displaying/filtering these new fields.