Add new modality constant flags for Contentnodes

[LianaHarris360]

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Description

To support the new Course -> Unit -> Pre-test -> Lesson(s) -> Post-test structure, we need to add new modality constant flags for content nodes to differentiate between Courses, Units, and Lessons.

Outcomes

• Introduce three new constants in the spec/constants-modalities.json file:

    MODALITY.COURSE
    MODALITY.UNIT
    MODALITY.LESSON