Allow organisation jobs link to be overridden

Organisation#job_url defaults to the civil service jobs search, but can
be overridden by setting the jobs_link_override_url attribute.

Story: https://www.agileplannerapp.com/boards/173808/cards/5251

- [ ] code review
- [ ] PO acceptance
"Jobs link override URL" sounds very technical. Would "Custom jobs link", or something similar, be better?