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
for some odd reason, this link_to doesn't work as we expect it to. the link href is the organisation's path instead of custom jobs URL:
![screen shot 2014-08-18 at 14 00 01](https://cloud.githubusercontent.com/assets/230074/3949191/1493ca2e-26b2-11e4-8ec4-d229ebcd3ed8.png)