Allow organisation jobs link to be overridden

Organisation#job_url defaults to the civil service jobs search, but can
be overridden by setting the custom_jobs_url attribute.

Story: https://www.agileplannerapp.com/boards/173808/cards/5251

- [ ] code review
- [ ] PO acceptance
Good spot. I always forget that `link_to` doesn't behave like `mail_to` when you pass it a single parameter... Fixed.