title: Message framework: Add message when saving Page class in Wagtail
body: <p>So I would like to check if a certain field has changed and if so, display a info-message to the user that they should review other datapoints. I feel that the Django messages framework should be ideal for this, but I cant find where to hook in to the wagtail Page class to get this to work.</p>
  
  <p>Right now I have the check on the save method for the page like below, but I do not have access to the request object here, so I cant use massage.add_message (since this uses the request object as a parameter). </p>
  
  <p>What am I missing?</p>
  
  <pre><code>    def save(self, *args, **kwargs):
      super().save(*args, **kwargs)
  
      if self.pk is not None:
          original = EventDetail.objects.get(pk=self.pk)
          if (
              original.start_date != self.start_date
              and self.messages.all().count() &gt; 0
          ):
              messages.add_message(
                  request,
                  messages.INFO,
                  _(
                      "You have just changed starting time on an event that contains messages, please review the times for the message sends."
                  ),
              )
  </code></pre>
  
  <p><strong>Edit</strong> after the reply by <strong>@gasman</strong>, my new before_edit hook now looks like this, can this be done in a more pythonic way? Seems a bit of a hack to be compare with the request, but I cant see any other solution?</p>
  
  <pre><code>@hooks.register("before_edit_page")
  def before_edit_page(request, page_object):
      if isinstance(page_object, EventDetail):
          if request.POST:
              original = EventDetail.objects.get(pk=page_object.pk)
              if (
                  page_object.live == True
                  and request.POST["start_date"]
                  and datetime.datetime.strptime(
                      request.POST["start_date"], "%Y-%m-%d %H:%M"
                  )
                  != original.start_date
                  and page_object.messages.filter(sent=False).count() &gt; 0
              ):
                  messages.add_message(
                      request,
                      messages.WARNING,
                      _(
                          "You have changed the start time of an event that has unsent messages. Please review the timing of the messages."
                      ),
                  )
  </code></pre>