Student email notifications use student username instead of email address

[brianhogg]

Reproduction Steps

1. Register a new student and check your email, when there are no Username form fields

Expected Behavior

Welcome email contains email address beside "Your Login:"

Actual Behavior

Email contains username, which appears to be the first part of the email address.

This issue has be recreated:

• [ ] Locally
• [ ] On a staging site
• [ ] On a production website
• [ ] With only LifterLMS and a default theme

[brianhogg]

The notification class is using a now deprecated lifterlms_registration_generate_username option, instead of the LLMS_Forms::instance()->are_usernames_enabled() method that things like "Lost password?" use to decide whether to ask for email address or username. If we just switch to using that flag instead it'll at least work the same as the Lost Password form.