title: allow_tags=True does not render <form> tag in django admin
body: <p>I want to display a form in the list_display in django admin, but I'm facing this problem:</p>
  
  <p>when I define something like this:</p>
  
  <pre><code>class MyModelAdmin(admin.ModelAdmin):
      list_display = ('foo', 'pagar_pase')
  
      def pagar_pase(self, obj):
          return """&lt;form action="." method="post"&gt;Action&lt;/form&gt; """
      pagar_pase.description = 'Testing form output'
      pagar_pase.allow_tags = True
  </code></pre>
  
  <p>and the result is Action without  tags, any ideas how to solve this?</p>
  
  <p>thanks</p>