Migrate AssessmentItem data to proper JSON fields

Builds upon https://github.com/learningequality/studio/issues/897 and https://github.com/learningequality/studio/pull/3901

Desired behavior

Proper resolution of this will involve several steps:

• Converting answers, and hints fields into a jsonfield JSONField (not a proper postgres JSONField to avoid a complex migration for now).
• To ensure proper interaction with ValuesViewsets, we should copy the extension of the jsonfield JSONField that is done in Kolibri here: https://github.com/learningequality/kolibri/blob/release-v0.14.x/kolibri/core/fields.py#L102
• This will then require updating of the frontend to expect JSON, as opposed to a string for these fields https://github.com/learningequality/studio/issues/1855

By itself, these steps would at least be sufficient to ensure that the incoming data is validated as JSON on save.

However, as the data will now be stored in IndexedDB in the frontend as JSON, any edits to them will be recorded by keypath, rather than the entire text field changing. This has the advantage of giving more granular edits, but means we need to update our serializers to properly handle this.

• Some original work will be required to be done, probably using the DotPathValueMixin, and in a similar way to the TagField used in the ContentNode serializer.
• The way that the data is serialized to the frontend should also be updated in order to be sent as a map (see e.g. tags for contentnodes), however, as there is additional data rather than just a tag name, a more complex method would be required.

Current behavior

Data is stored as text but is validated as JSON in https://github.com/learningequality/studio/pull/3901

Start by reading issue 897 and pull request 3901, then inspect the AssessmentItem fields and the referenced Kolibri fields.py extension. Review DotPathValueMixin and the TagField and content-node serializer examples in common.py and contentnode.py. Done means answers and hints are stored and validated as JSON, with frontend serialization and keypath edits handled correctly.