Follow-up from "WIP: Resolve "New user flow for SSOing into a GitLab.com group""

The following discussion from !10338 (merged) should be addressed:

• @jamedjo started a discussion: (+2 comments)

  Minor frontend bug: filling this form out quickly and pressing enter on the keyboard displays 'Username is already taken.' before successful sign up.

  I think because it is asynchronous and does the check after the user has already been created. Perhaps we should cancel the async validation after form submit?

  UsernameTakenOnSignup

We should unbind events from triggered by the username validator when we submit a form.