js: Don't trigger beforerender or rendered for discarded content

A listener for `rendered` should not need to check if the content
really changed. Though, if such a listener is not triggered anymore,
`beforerender` listeners also must not be triggered, as they might
assume that the content is really being updated and their accompanied
`rendered` listener is triggered. (e.g. input-enrichment.js)

This might be breaking change and any `Behavior.renderHook` implementation
needs to be checked against it. Potentially also in third party modules.
As if such an implementation updates the container on its own,
`beforerender` listeners only have access to the updated container after
this change, while they had access to the original beforehand.

`rendered` listeners should not be that much affected, as for them the
change results in the same behavior as if no update has ever been
scheduled for the container.

fixes #5056

public/js/icinga/loader.js

@@ -873,7 +873,7 @@
  }
  })
  } else {
- this.renderContentToContainer(
+ let rendered = this.renderContentToContainer(
  req.responseText,
  req.$target,
  req.action,
@@ -882,6 +882,10 @@
  req.autosubmit || autoSubmit,
  req.scripted
  );
+
+ if (! rendered) {
+ req.discarded = true;
+ }
  }
 
  if (oldNotifications) {
@@ -905,6 +909,14 @@
  req = reqOrError;
  }
 
+ req.$target.data('lastUpdate', (new Date()).getTime());
+ delete this.requests[req.$target.attr('id')];
+ this.icinga.ui.fadeNotificationsAway();
+
+ if (req.discarded) {
+ return;
+ }
+
  // Remove 'impact' class if there was such
  if (req.$target.hasClass('impact')) {
  req.$target.removeClass('impact');
@@ -957,10 +969,6 @@
  }
  }
 
- req.$target.data('lastUpdate', (new Date()).getTime());
- delete this.requests[req.$target.attr('id')];
- this.icinga.ui.fadeNotificationsAway();
-
  var extraUpdates = req.getResponseHeader('X-Icinga-Extra-Updates');
  if (!! extraUpdates && req.getResponseHeader('X-Icinga-Redirect-Http') !== 'yes') {
  $.each(extraUpdates.split(','), function (idx, el) {
@@ -1253,8 +1261,6 @@
  }
  }
 
- $container.trigger('beforerender', [content, action, autorefresh, scripted, autoSubmit]);
-
  var discard = false;
  $.each(_this.icinga.behaviors, function(name, behavior) {
  if (behavior.renderHook) {
@@ -1272,6 +1278,8 @@
  });
 
  if (! discard) {
+ $container.trigger('beforerender', [content, action, autorefresh, scripted, autoSubmit]);
+
  if ($container.closest('.dashboard').length) {
  var title = $('h1', $container).first().detach();
  $container.html(title).append(content);
@@ -1337,6 +1345,8 @@
 
  // Re-enable all click events (disabled as of performance reasons)
  // $('*').off('click');
+
+ return ! discard;
  },
 
  /**