reduce heroku slug size

[Hamms]

Looks like CurriculumBuilder has become too large of an application. My most recent change seems to have pushed us over some kind of internal limit. The build log says:

 !     Compiled slug size: 534.9M is too large (max is 500M).
 !     See: http://devcenter.heroku.com/articles/slug-size
 !     Push failed

This PR represents my attempts to get us back under the limit. In addition to the code changes contained within, I also purged the build cache as recommended by https://devcenter.heroku.com/articles/slug-compiler#slug-size

Links

• spec
• jira

Testing story

Reviewer Checklist:

• Tests provide adequate coverage
• Code is well-commented
• New features are translatable or updates will not break translations
• Relevant documentation has been added or updated
• User impact is well-understood and desirable
• Pull Request is labeled appropriately
• Follow-up work items (including potential tech debt) are tracked and linked