[PR jobs] Check if the organization is whitelisted

When submitting pull requests jobs, PR URLs might be from external systems (or on your own fork, or...) so before trying to send an update request to that PR on github, check that we have rights to do so.

See https://jenkins.plone.org/job/pull-request-5.2-3.6/6/console

Thanks to @petschki to report it on https://community.plone.org/t/8299/6