Add option to run CORGI jobs on forked book repos.

[ckline-tryptic]

Story

As a tester, it would be very useful to be able to fork a book repo and run CORGI jobs against it.

Acceptance Criteria

• [ ] osbook repos are forkable
• [ ] corgi accepts full github repo urls in the Collection ID field

[m1yag1]

Existing issue for securing CORGI backend in case we want to edit / add comments: 1172

[m1yag1]

Issue moved to openstax/cnx #1561 via ZenHub