[REFERENCE] fix: ensure course run key is passed in legacy enrollment urls

[adamstankiewicz]

While further triaging https://2u-internal.atlassian.net/browse/ENT-9770, we raised the question of WHY learners are seemingly visiting the legacy enrollment_url with a top-level course key vs. an explicit course run key.

When this occurs, it seems the InvalidKeyError is raised, regardless of the /enroll suffix being included in the course identifier passed to CourseKey.from_string(...).

This PR demonstrates a possible fix for what seems like a bug where generating an enrollment_url for a ContentMetadata record with content_type="course" results in the top-level course key being included instead. The proposed fix here would be to resolve the advertised course run key to use in the enrollment_url instead of the top-level course key, which should also help mitigate the InvalidKeyError error.

In theory, no InvalidKeyError errors should be raised from this legacy enrollment view.

Post-review

• Squash commits into discrete sets of changes
• Ensure that once the changes have been deployed to stage, prod is manually deployed