title: AbstractBaseUser 'is_staff' field being omitted during syncdb
body: <p>I've decided to create a custom user model for an app by extending Django's AbstractBaseUser along with the PermissionsMixin class. For some reason when I run syncdb or sqall, the <code>is_staff</code> field on my model is omitted from the query. As you know, the <code>is_staff</code> field is required to be able to access the Admin interface. Here's my code snippet(Django 1.5.8):</p>
  
  <pre><code>from django.contrib.auth.models import ( \
  AbstractBaseUser, BaseUserManager, PermissionsMixin)
  
  #models.py snippet
  class FooUser(AbstractBaseUser, PermissionsMixin):
      #foo fields here...
      is_active = models.BooleanField(default=False, verbose_name='account is activated')
      is_staff = models.BooleanField(default=False, verbose_name='staff account')
  </code></pre>
  
  <p>I've looked at the source code of the two classes and neither of them implement the <code>is_staff</code> field so I'm not sure what the issue is. The only reason why I'm using AbstractBaseUser instead of AbstractUser(which has the <code>is_staff</code> field) is because I don't need or want the first, last name fields in my user model. Adding other fields such as <code>is_admin</code> worked just fine but this particular field just won't budge.</p>