This PR adds username in the email sent when users are created and enrolled in a course at the same time with `create_and_enroll_user`. (Which should be the case when using the `ALLOW_AUTOMATED_SIGNUPS` feature.)
*Sandbox URL*: TBD - sandbox is being provisioned.
*Merge deadline*: "None"
*Author notes and concerns*:
[ ] edX reviewer[s] TBD