add absolute url (also for 'view on site' in admin app)

fet2020/posts/models.py

@@ -206,6 +206,9 @@ class Post(models.Model):
     def url(self):
         return reverse("posts.show", kwargs={"id": self.slug})
 
+    def get_absolute_url(self):
+        return self.url
+
     def clean(self):
         if self.event_end and self.event_end < self.event_start:
             raise ValidationError(_("Das Ende des Events liegt vor dem Beginn."))