SCOPING: Diff tool for study submission review

Pain point: Before studies are actually deployed on Lookit, they have to be approved by an admin, which reserves an opportunity for us to check for compliance with terms of use, help researchers ensure instructions are clear, etc. Studies have to be re-approved after changes are made. But there's currently no way for an admin to tell what has changed, which would allow vastly expedited review in the case where the researcher fixed a typo or changed the age range, allowing us to focus energy on cases where new code has been introduced, etc.

Acceptance criteria: When reviewing a submitted study, a Lookit admin can see what has changed since the last approved version (if there is one) and see a history of actions taken on the study (e.g. edits/state changes). Either when saving changes to a study or when submitting, a researcher can provide a note about the purpose of the changes (like a commit message).

Implementation notes/Suggestions: Changes may have been made to any of the fields on the study model - e.g. purpose, description, title, eligibility criteria, JSON doc, commit SHAs. For all except the JSON doc simply displaying the previous and current versions of any changed fields would be fine. For the JSON doc some sort of actual diff output would be helpful if possible since often changes will be just to a few lines.

Start by locating the study model and the submission, approval, saving, and state-change entry points. Review how prior approved versions, changed fields, JSON documents, commit SHAs, and researcher notes are represented. Done means an admin can compare the current submission with the last approved version, inspect study action history, and see the purpose of researcher changes.