Added a email_subject function to Sale. #5996

We have the following options for implementing the flexibility to change email subject in downstream modules -:

1. We could add the subject as a field in Sale Configuration. The Tryton user then sets the subject from the client.
2. We could add a method for generating the email subject, initially having only a generic subject message, which can be overriden in downstream modules. It will take as argument a `purpose`.

This PR implements option 2.