Improve validation of openedx_content to prevent creation of corrupt states

[bradenmacdonald]

This PR implements tests for the issues identified in #463 .

Two issues are unaddressed as they are API design choices: create_publishable_entity_version() and create_publishable_entity() can both create either a useless or an invalid state if they are used on their own without corresponding [Component/Container/etc.] versions.

[openedx-webhooks]

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