Postajob admin fixes [PD-741]

We no longer guess what company to use on Django admin pages
There is a note on job posting that making the price nonzero can have bad effects if the company doesn't have Authorize.net details

I believe this is all that is needed to make the admin functional and everything I've tried has worked. @almathew should probably be one of the reviewers when she comes back, though.

All tests pass.