title: Create custom user form
body: <p>On Django Admin, the default 'create user' form has 3 fields: username, password and confirm password.</p>
  <p>I need to customize the create user form. I want to add the <code>firstname</code> and <code>lastname</code> fields, and autofill the username field with <code>firstname.lastname</code>.</p>
  <p>How can I do this?</p>