Revise categories into job roles

[jace]

The current system of categories isn't very useful and needs revision. "Programming" et al are just too broad. For candidates looking for, say, a CTO role, searching for "CTO" does not help to distinguish between "CTO role" and "report to the CTO".

Instead of introducing a new "role" type, we could redefine the category to deal with this, and optionally make jobpost<-->category many-to-many so multiple categories can be selected.