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

plus some cleanup and removal of unused code

related to these two other pull requests:
* https://github.com/GigaOM/go-syncuser/pull/2
* https://github.com/GigaOM/gigaom/pull/4640

after deploying but before activating this plugin, we need to carry out the following steps:
  1. Update our webhook endpoint on Mailchimp and update a query var name (rename the `go-mcsync-webhook` action to `go-mailchimp-webhook` and the `mcsyncwhs` query var on the webhook url to `mailchimpwhs`)
  1. rename the `GO_MCSYNC_LIST_*` defines in wp-config.php to `GO_MAILCHIMP_LIST_ID` and `GO_MAILCHIMP_LIST_NAME`.
  1. rename the following user meta keys in mysql db's wp_usermeta table:
    1. `go_mcsync_cronned` => `go_mailchimp_cronned`
    1. `go_mcsync_do_not_email` => `go_mailchimp_do_not_email`
    1.  `go_mcsync_subscriber_info` => `go_mailchimp`
    1.  `go_mcsync_user_status` => `go_mailchimp_user_status`
  1. add the following MailChimp merge tags/vars, all of type "text", to the "General Users" list:
    1. `go_age` / `MMERGE13`
    1. `go_gender` / `MMERGE14`
    1. `go_zip` / `MMERGE17`
    1. `go_education` / `MMERGE18`
    1. `go_household_income` / `MMERGE23`
    1. `go_high_net_worth` / `MMERGE24`
    1. `go_occupation` / `MMERGE25`
  1. update at least the [action name](https://github.com/GigaOM/gigaom-plugins/blob/fc6c132cc1f2c4aa05abad1707500a8bc08eaafc/go-mcsync-bstat/components/class-go-mcsync-bstat.php#L25) in [go-mcsync-bstat](https://github.com/GigaOM/gigaom-plugins/tree/master/go-mcsync-bstat) (but we might as well rename the plugin to `go-mailchimp-bstat`).

see https://github.com/GigaOM/legacy-pro/issues/3526
yeah, @zbtirrell! (https://github.com/GigaOM/go-mcsync/commit/99ecf4e3cbbbf4d0768d0c41afc908f7bf4d3b6c)