Throw RuntimeError if MAILCHIMP_API_KEY is None
Set update_existing to True to prevent ListAlreadySubscribed error
Add error handling and fix view function responses

- Make mailchimp_get_endpoint return an empty response
  with a 200 status code
- Have user_choose_mailing_lists return mailing lists
  a user is subscribed to and throw a 400 error if
  json_data is None
- In sync_data_from_mailchimp, raise error if user
  does not exist
Remove /static/public/js/dataverse/files.js

Files in /static/public should not be versioned
Remove remaining files in website/static/public/js
Update sync_email script

- Use logging
- Add update_existing=True parameter to
  lists.batch_subscribe
- Fix test_subscribe_users_called_with_correct_args
Use (un)subscribe.delay when USE_CELERY is True
Create knockout view model for user settings email page
Move mailchimp helpers from auth.utils to separate file
Add docstrings to view functions