Historically image files have not been associated with their assessment items

[rtibbles]

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Observed behavior

https://github.com/learningequality/studio/pull/5072 resolves a long standing bug whereby images were being incorrectly detected in assessment item json/markdown fields. Going forwards, this should now be handled properly, but for any assessment item images uploaded prior to this fix, the files are not properly tracked.

This may mean if the assessment items are subsequently updated, that either the user suddenly is responsible for a large number of image files, or possibly that updates will fail if the file does not have an associated file object in the database.

Expected behavior

Before running this migration, we need to fix this issue: https://github.com/learningequality/studio/issues/3306 and also ensure that users are not responsible (storage wise) for images in imported assessments that they have not edited.

We should:

• Run a migration to properly associate file objects with all assessment items.