Add ability to hide username field when creating new users

Problem/Motivation

In order to get one step further of masquerading the fact that a username even exists I think it could be valuable for the module to completely hide the username field when creating a user

Steps to reproduce

Install email_registration_username
Create new user with admin

Proposed resolution

Add a checkbox and a configuration value to hide the username in the create user form.
When enabled the form behaves the same as leaving the username

Remaining tasks

Agree on solution
Code

User interface changes

New config option, disabled by default. When enabled it hides the usernam

API changes

None that I'm aware of