title: Django - disable model editing
body: <p>Is there a way, hopefully without breaking admin, to disable editing existing model instances on the ORM level?</p>
  
  <p>I'm not talking about removing 'Save' and 'Save and continue' buttons from templates - there should be no operations that can change the values of a committed instance of a model. </p>
  
  <p>Preferably, the 'Save As' option should work instead. </p>