Allow superusers to change the Contributor of a Source

[jwalgran]

Overview

The ability to transfer the ownership of a "public list" to a contributor after they sign up was lost when we transitioned to using the Source model and made the contributor field read-only on the admin.

Connects #900

Demo

Testing Instructions

This assumes you have run ./scripts/resetdb

• Register a new account.
• Log in as c1@example.com
• Browse http://localhost:8081/admin/api/source/2/change/. Verify that the Contributor field is a dropdown list.
• Make note of the old contributor, select a different contributor, and save the Source. Make note of the new contributor.
• Browse http://localhost:8081/?contributors=2 and verify that there are no results.
• Search for the newly registered contributor account and verify that there are results.

Checklist

• fixup! commits have been squashed
• CI passes after rebase
• CHANGELOG.md updated with summary of features or fixes, following Keep a Changelog guidelines

[rajadain]

+1 tested, works as described!

[jwalgran]

Thanks for the review.