Handle mismatching datasets for exams and lessons to prevent syncing issues

[bjester]

Background

Lesson and exam models have foreign references to the user model, and if those models are created by a user who was in a different facility (like a superuser), then the models would fail to deserialize when synced

Summary

• Makes creator, created_by, and assigned_by nullable
• Adds enforcement to still require users for the above fields, but allows it to be null for the cross-facility superuser edge case
• Adds additional enforcement to ensure assignment datasets match datasets of their referenced exams/lessons
• Adds migrations to fix un-syncable lessons and exams caused by the issue Don't set user as created_by for lesson when superadmin from Facility B creates a lesson in Facility A #7453

References

Resolves: #7453
Builds off: #7887
Ref: #7859

Reviewer guidance

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[rtibbles]

I had to read a bit harder to parse what was happening in the upgrade steps - but nothing blocking. Code makes sense and tests cover it!

[rtibbles]

This took me a couple of reads to get my head around. I think I understand what is going on though.

The dataset_id of the Exam has previously been set from the creator, and so if there is a mismatch between the dataset_id of the Exam and the dataset_id of the collection it belongs to, then we have discovered our problem exams.

[bjester]

Yeah the dataset inference was changed in #7887 so this migrates that change, like you said where the collection dataset doesn't match the exam's. This wasn't as simple as I wanted, and as simple as the query could have been, but Django constrained me a little bit.

[rtibbles]

Presumably this is now relying on the migration above having happened?

[bjester]

Yeah that's a good note, it does depend on that to achieve the end result we want