title: How to allow sorting in the Django admin by a custom list_display field, which doesn't have a DB field nor annotatable
body: <p>I have a custom list_display field which is responsible for a column of integers in one of my admin pages.</p>
  
  <p>I need to allow staff members to sort according to it.</p>
  
  <p>There's a solution for how to acheive that if the integer represents a count/average/etc of some DB field, which is not the case for me.</p>
  
  <p>[ the solution for that case is here: <a href="https://stackoverflow.com/questions/2168475/django-admin-how-to-sort-by-one-of-the-custom-list-display-fields-that-has-no-da">Django admin: how to sort by one of the custom list_display fields that has no database field</a>
  ]</p>
  
  <p>Any ideas how I can achieve this sorting without actually creating and maintaining a DB field for the values?</p>