add mailchimp API
auto add new users to old list
add mailchimp subscribe function
subscribe to mailchimp CommCare Users if selected at signup
remove email opt out from account settings management
catch and log mailchimp errors
store signup choice
do not log already subscribed errors
add mailchimp unsubscribe function
resubscribe mailchimp recipient when email is changed
fix error
only unsubscribe a changed email if no other user has the same email
do not log already subscribed error when email is changed
correctly check for other users with the same email
correctly hide already subscribed errors
Merge pull request #3424 from dimagi/mailchimp3

use mailchimp for product updates