Require admin approval before preview if using own code

@kimberscott is already working on this.

Since Mar 2, 2020.

**TL;DR**: Now that previewing a study actually saves data to the database, we should check that custom code used will not cause any problems.

**Narrative**

"As an Admin, I would like to minimize the possibility of security or functionality problems caused by previewing custom frameplayer code - e.g., saving 100MB per response or trying to edit previous responses from the same participant."

**Acceptance Criteria**

- [ ] Updated state diagram created and added to docs.
- [ ] Whenever the frameplayer repo or version are changed when saving a study, and the repo is not the default frameplayer we maintain, the study should go into a state where it is not possible to preview until approval. Building the frameplayer may be allowed or disallowed in this state.
- [ ] The study researcher should then be able to "submit" the study for preview-only approval, a separate state from submitting for approval to run. Submission should trigger an email to admins.
- [ ] An admin (same people as can approve studies to run) should be able to approve/reject the study for preview when it has been submitted for preview. This should trigger an email to study researchers.
- [ ] The preview proxy should not work when the study is in an unapproved-for-preview state. The preview view should show a clear warning/indication of why it's not possible to preview and what to do.
- [ ] Study researchers NOT using custom code should not see the option to submit for preview approval and should not require approval to preview. Their study may be in a preview-approved state immediately upon saving, for instance.

**Implementation Notes**

See https://lookit.readthedocs.io/en/develop/implementation-workflow.html