[PD-839] my.jobs - posting - site package selection UX improvements

* Trying to visit any of the job posting related URLs as the company will show a screen prompting you to contact your member representative
* Trying to visit the purchased jobs pages works as usual
* Trying to view job listings for a company who doesn't have site packages will 404

I used `Company.sitepackage_set` for this since `Company.site_package` is unreliable (eg, disowning a site package doesn't update the `site_package` reference) and its existence seems redundant. Please correct me if I'm wrong about this point.