Improve PolymorphicParentAdmin, simplify, fix templates

During the development of django-polymorphic-tree it was discovered that
the PolymorphicParentModelAdmin could actually be made much simpler.
It features a `child_models` attribute now, so there is very little code
needed to actually implement a polymorphic admin now.

Also found various issues which are together fixed in this commit for
pulling.

polymorphic/templates/admin/polymorphic/delete_confirmation.html

@@ -6,9 +6,6 @@
      <a href="../../../../">{% trans "Home" %}</a> &rsaquo;
      <a href="../../../">{{ app_label|capfirst|escape }}</a> &rsaquo;
      <a href="../../">{{ opts.verbose_name_plural|capfirst }}</a> &rsaquo;
-     {% for p in parent_object.breadcrumb %}
-       <a href="../{{ p.id }}/">{{ p.title }}</a> &rsaquo;
-     {% endfor %}
      <a href="../">{{ object|truncatewords:"18" }}</a> &rsaquo;
      {% trans 'Delete' %}
 </div>