Fixed #8261 -- ModelAdmin hook for customising the "show on site" button

``ModelAdmin.view_on_site`` defines wether to show a link to the object on the
admin detail page. If ``True``, cleverness (i.e. ``Model.get_absolute_url``) is
used to get the url. If it's a callable, the callable is called with the object
as the only parameter. If ``False``, not link is displayed.

With the aim of maitaining backwards compatibility, ``True`` is the default.

tests/admin_views/models.py

@@ -717,3 +717,19 @@ class EmptyModelHidden(models.Model):
 
 class EmptyModelMixin(models.Model):
     """ See ticket #11277. """
+
+class State(models.Model):
+    name = models.CharField(max_length=100)
+
+class City(models.Model):
+    state = models.ForeignKey(State)
+    name = models.CharField(max_length=100)
+
+class Restaurant(models.Model):
+    city = models.ForeignKey(City)
+    name = models.CharField(max_length=100)
+
+class Worker(models.Model):
+    work_at = models.ForeignKey(Restaurant)
+    name = models.CharField(max_length=50)
+    surname = models.CharField(max_length=50)