[SE-2892] Fix issues with instance reference admin form and creator assignment

Created by: fferrin

This PR:

• changes how the inlines instance references of the OpenedX instance are displayed on the admin page. Now you should see them expanded.
• allows you to save an instance without providing and owner or creator. By default, it sets these fields to the currently logged user.

JIRA ticket: SE-2892

Testing:

• Go to an OpenedX instance on the admin page, edit it and verify that the instance reference at the bottom of the form is expanded
• Save a new Instance without providing an owner or created. You should be able to do it.

Reviewers

• @mtyaka