refactor!: remove learner_downloadable field

[Ian2012]

This PR removes the learner_downloadable field.

Closes: #237

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Thanks for the pull request, @Ian2012!

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[ormsbee]

Looks great! I have a minor request because a change in requirements on my part. Thank you!

[ormsbee]

Code looks good, but please bump the version to 0.17.0