Job Board: amendments to submission form, admin and DB schema

Enables job submission form to be usable without company being set up before hand by making dependencies on Company model optional... Adds company_name and company_description to Job model instead. Slight alterations to template to prioritise these fields over similar Company model fields.

Resolves most of https://github.com/python/pythondotorg/issues/246

Note: `./manage.py jobs migrate` is required...