go-mailchimp refactor

* Added the `GO_MailChimp_Admin` class to manage all the admin-related code
* removed the "MailChimp" synchronization status column from the users admin dash page
* renamed the webhook handler to conform to our coding standards
* remove the two copies of `log()` function and use the `go_slog` action directly

see https://github.com/GigaOM/legacy-pro/issues/3828