title: Add non model based URL via AdminSite.get_urls not working
body: <p>I tried to add an non model based URL to an admin page using get_urls member function:</p>
  
  <pre><code>from django.contrib.admin.sites import AdminSite
  from django.conf.urls import url
  from myapp.models import Bar
  
  class StaffAdmin(AdminSite):
      site_header = 'My App'
      site_title = site_header
      def get_urls(self):
          urls = super(StaffAdmin, self).get_urls()
          my_urls = [
              url(r'^foo/$', self.foo_view, name='foo'),
          ]
          return my_urls + urls
      def foo_view(self, request):
          context = dict(
              self.admin_site.each_context(request),
              key=value,
          )
          return TemplateResponse(request, 'foo_view.html', context)
  
  staff_admin_site = StaffAdmin()
  staff_admin_site.register(Bar)
  </code></pre>
  
  <p>Bar shows up OK on the staff admin page, but foo is missing.</p>
  
  <p>Reference: <a href="https://docs.djangoproject.com/en/dev/ref/contrib/admin/#django.contrib.admin.ModelAdmin.get_urls" rel="nofollow">https://docs.djangoproject.com/en/dev/ref/contrib/admin/#django.contrib.admin.ModelAdmin.get_urls</a></p>