Freebird: go-subscriptions

This is the old `go-subscriptions` plugin minus recurly-specific stuff.

see https://github.com/GigaOM/legacy-pro/issues/4125
Actually, a bunch of this assumes use of Mandrill. Let's talk about how to divide responsibility between go-subscriptions and go-local-subscriptions. Something like:

```php
$email = array(
  'to' = $user['email'],
  'subject' = 'Welcome to '. get_bloginfo( 'name' ),
  'content' = 'Hot damn! This is your welcome email!',
  'headers' = array(),
);

$email = apply_filters( 'go-subscriptions-welcome-email', $email, $user );