Add new field contractType to Job Postings

[MeZKaL]

Adds a new field on the Job Posting form to indicate if the new posting's Contract Type is Direct Employment, Employee of Record or Contractor.

Disclaimer 1: Not tested yet, gonna find out how to do it now 🙈 Disclaimer 2: I think this my first ever OSS contribution via a fork/PR. Disclaimer 3: First time "writing" go (if you can call writing copy&pasting the code in the side)