* Download CSV link shown only if waffle switch enable_learner_download is on.
* Download CSV link is styled like a button, with role=button, and href set to
  initial Download CSV link.
* Downloaded report reflects active search filters, and a configured fields
  list from settings.LEARNER_API_LIST_DOWNLOAD_FIELDS, and sets page_size from
  the total_records returned by the JSON fetch.
* Adds a learner list CSV endpoint to the learner_analytics_api, which proxies
  plain text responses from the Analytics Data API
* Reduces code duplication by refactoring BaseLearnerApiView and its subclasses
  to handle different content types