🗒️ Checklist

1. run linter locally
2. update developer docs (API, README, inline, etc.), if any
3. for user-facing doc changes create a Zulip thread at #Support Docs Updates, if any
4. draft PR with a title <type>(<scope>)<!>: <title> DEV-1234
5. assign yourself, tag PR: at least Front end and/or Back end or workflow
6. fill in the template below and delete template comments
7. review thyself: read the diff and repro the preview as written
8. open PR & confirm that CI passes & request reviewers, if needed
9. delete this section before merging

💭 Notes

Port selected tests over from test_submission_extras_api_post.py in the old app. Which tests to port were chosen based on whether or not they applied in the new format and if they already had a reasonable equivalent, which many did.
Also cleans up some old tests that were failing or had misleading comments.