the go-syncuser plugin

A new plugin to coordinate user data synchronization. This plugin calls the `go_syncuser_user` action with the id of the user being modified and the type of operation being done to the user ('add', 'update', 'delete') so the listener can trigger any synchronization with other data repos (MailChimp, Marketo, etc.).

related to [this pull request](https://github.com/GigaOM/gigaom/pull/4597)

see https://github.com/GigaOM/legacy-pro/issues/3525
Do we just want to call `$this->sanitize_user(...)` here?