title: Can't edit but can add new inline in Django admin
body: <p>Here are my models</p>
  
  <pre><code>class Note():
      note = models.TextField(null=False, blank=False, editable=True)
      user = models.ForeignKey(to=User, null=True, blank=True)
  
      content_type = models.ForeignKey(ContentType, on_delete=models.CASCADE)
      object_id = models.PositiveIntegerField()
      content_object = GenericForeignKey("content_type", "object_id")
  </code></pre>
  
  <p>And an inline I created this model to incorporate in any admin is below</p>
  
  <pre><code>class NoteInline(GenericTabularInline):
      model = Note
      extra = 0
  </code></pre>
  
  <p>What I need here is, I want to see all the current notes but don't want the logged in user to edit them. At the moment user can edit old and add new. So here is what I did,</p>
  
  <pre><code>class NoteInline(GenericTabularInline):
      model = Note
      extra = 0
  
      def get_readonly_fields(self, request, obj=None):
          if obj and 'change' in request.resolver_match.url_name:
              return ['note', 'user', ]
          else:
              return []
  </code></pre>
  
  <p>But now if user adds new note he sees a disabled (not editable) note text ares. However user can see old fields not editable.</p>
  
  <p>How to implement this functionality?</p>