ported MailChimp-specific functions from go-mcsync to go-mailchimp

plus some cleanup and removal of unused code

see https://github.com/GigaOM/legacy-pro/issues/3526
Good point about `GO_MAILCHIMP_LIST_ID` and `GO_MAILCHIMP_LIST_NAME`, but is that too specific to our case to be in the public repo? Perhaps the comments should live in https://github.com/GigaOM/gigaom/pull/4640 ?