validate level key and re-enable script edit gui test

[davidsbailey]

Follow on to #37236 . Adds validations and tests ensuring that we can compute key for all levels, and re-enables the script gui edit page ui test scenario. I was not able to track down why a bunch of PeerReview objects would have levels without game set properly, so I went a bit overboard with precautions by running validations on create as well as update and also adding a safety check against problematic test fixtures.

Testing story

After deleting problematic Level and PeerReview objects for the test machine, the Level.key_list command now completes there successfully. That combined with the added validations and tests should help ensure that Level.key_list does not break again in the future.

Reviewer Checklist:

• Tests provide adequate coverage
• Privacy and Security impacts have been assessed
• Code is well-commented
• New features are translatable or updates will not break translations
• Relevant documentation has been added or updated
• User impact is well-understood and desirable
• Pull Request is labeled appropriately
• Follow-up work items (including potential tech debt) are tracked and linked