title: Multiple USERNAME_FIELD in django user model
body: <p>My custom user model:</p>
  
  <pre><code>class MyUser(AbstractBaseUser):
      username = models.CharField(unique=True,max_length=30)
      email = models.EmailField(unique=True,max_length=75)
      is_staff = models.IntegerField(default=False)
      is_active = models.IntegerField(default=False)
      date_joined = models.DateTimeField(default=None)
  
      # Use default usermanager
      objects = UserManager()
  
      USERNAME_FIELD = 'email'
  </code></pre>
  
  <p>Is there a way to specify multiple <code>USERNAME_FIELD</code> ? Something like <code>['email','username']</code> so that users can login via email as well as username ?</p>