New fields for Jobs posts: categories/tags, company-size, full-time/internship

All of these would be valuable.
The full-time/internship distinction would let us make the end-date field optional for full-time positions. This is based on feedback from unmanned innovations.

We should also maybe keep track of the number of days a listing has been up, and have them expire after a certain amount of time, say, 6 months.
