Create customer case study migration script

Closes #2862 (closed) + Related to #2861 (closed)

Migration script isn't working at 100%, but all case studies are now in Contentful!

• Case Studies: https://app.contentful.com/spaces/xz1dnu24egyd/entries?id=FoyM8Fgav6FPG8CL&title=Overview&contentTypeId=caseStudy&order.fieldId=updatedAt&order.direction=descending&displayedFieldIds=contentType&displayedFieldIds=updatedAt&displayedFieldIds=author&entitiesTotal=78&page=0
• Templatized (un-changing) data: https://app.contentful.com/spaces/xz1dnu24egyd/entries/2YUEuoVflhGaUySCB3I7Lp