send welcome email as soon as possible after freebie invoicing

- ref: https://github.com/GigaOM/legacy-pro/issues/4541
can you add a comment explaining why we have to set the activation key again? (because `go_recurly()->subscribe_free_period()` now calls `go_subscriptions()->send_welcome_email()`, which calls `wp_set_password()`, which clears the user's activation key which we just generated.)