docs: def add_message(self, request, level, message_template,
                      message_context=None, extra_tags=''):
          """
          Wrapper of `django.contrib.messages.add_message`, that reads
          the message text from a template.
          """
          if 'django.contrib.messages' in settings.INSTALLED_APPS:
              try:
                  if message_context is None:
                      message_context = {}
                  message = render_to_string(message_template,
                                             message_context).strip()
                  if message:
                      messages.add_message(request, level, message,
                                           extra_tags=extra_tags)
              except TemplateDoesNotExist:
                  pass