title: Keep django messages until they are deleted
body: <p>How to keep django messages alive (in the django admin), until they are deleted by a further action?</p>
  
  <p>I use the django messages to remind the user of a required activity. The message should displayed until the required activity is done.</p>
  
  <p>For one view, I renew the message every time the change_view is called. But I don't think this is a solution for the whole django-admin.  </p>
  
  <p>admin.py</p>
  
  <pre><code>class InvitationAdmin(admin.ModelAdmin):
      def change_view(self, request, object_id, form_url='', extra_context=None):
          # keep massages alive
          answerSubQuestions = request.session.get('answerSubQuestions')
          #add massages
          if answerSubQuestions:
              for answerid in answerSubQuestions:
                  answer = Answer.objects.get(pk=answerid)
                  messages.warning(request, mark_safe('&lt;p style="float:left;line-height:29px;margin-right:15px;"&gt;Please add answer for subquestion: &lt;/p&gt;&lt;a href="%s" class="actionButton actionButtonBlue grp-button" onclick="return showAddAnotherPopup(this);"&gt;click here!&lt;/a&gt;'%reverse('changeAnswer', args=[answer.id,answer.question.getQuestionType()])))
          return super(InvitationAdmin, self).change_view(request, object_id, form_url, extra_context=extra_context)
  </code></pre>
  
  <p>I would like to have something like:</p>
  
  <pre><code>messages.warning(request, mark_safe('Message'), keepalive=True)
  </code></pre>