Move (un)subscribe_mailchimp to mailchimp_helpers.py
Add error message to user_choose_mailing_lists
Rename mailchimp_helpers.py to mailchimp_utils.py
Handle ListNotSubscribedError in view

- Re-raise as 400 error
- Client will not show error message if
  status code is 400.
Refactor user_notifications view function
Add sentry logging to NoResultsFound error in webhook endpoint
Make ajax request to get user list info

Remove use of passing data from the mako
context to the view model. Instead, make
an ajax request to get the data.