Use user email as username

### Proposed change
I would like to propose using the user email as username so that we would be able to remove one field (username) from the `signup` and improve the overall UX around the signup. That is a real use case for a project that I am currently working on at Anaconda.

I drafted a PR that can be found here https://github.com/jupyterhub/nativeauthenticator/pull/209

### Alternative options

### Who would use this feature?
Users interested on creating a new account

### (Optional): Suggest a solution
https://github.com/jupyterhub/nativeauthenticator/pull/209