Change reading for mastery_model

[AtKristijan]

Summary

Description of the change(s) you made

Fixes parts of #3339

I change the reading for mastery model and put both cases for reading.

Comments

I am not sure if I change everywhere the master_model, i search the project but could not find other ones.

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[rtibbles]

A few changes needed - to confirm you have caught all instances, have you searched for mastery_model in the Python files?

The other thing that is missing here is a unified representation from the ContentNode viewset, there have been no changes there, so it would currently be up to the frontend to also handle this data being present in either location.

[rtibbles]

Note that this still needs updates in the ContentNode viewset also.

[rtibbles]

These should be and not or here, otherwise you are trying to access .get from None, if exercise_data.get('option') is None.

I think if you fix this, the tests should pass, and we can merge this PR. We'll manage the rest in follow up work.

[AtKristijan]

This is changed