renamed the webhook handler to conform to our coding standard (with "_ajax" at the end)

replaced go-mailchimp's own log() function with go_slog action calls.

see https://github.com/GigaOM/legacy-pro/issues/3828
fixed an incorrect variable name
Merge branch 'master' of github.com:GigaOM/go-mailchimp into github-3828

Conflicts:
	components/class-go-mailchimp-api.php
moved admin-related functions into its own class/file
removed the MailChimp sync status column from the users admin page
removed some extraneous parens
Merge pull request #5 from wluo/github-3828

go-mailchimp refactor