Send registration link after Pro purchase

From a customer:

> One feedback: if a customer not registered and purchased the product using email address and cc information, then the customer not able to see the essays. On other words, purchase first and then login has some issue (my case) and that needs a manual fix just like what you had to do to fix my account.

Possible fix:

After successful order, send an email with a link that contains the order id. Clicking the link from any browser will allow completing registration (one time only, so it can't be shared to create unlimited Pro accounts).

If it's already been used, then redirect them to the login page.