[FEATURE] Forum: Allow data schema for "TinyMCE" images

See: https://mantis.ilias.de/view.php?id=47421
(cherry picked from commit 8a76c94)

Author: mjansenDatabay

components/ILIAS/Forum/classes/Purifier/class.ilHtmlForumPostPurifier.php

@@ -28,7 +28,7 @@ protected function getPurifierConfigInstance(): HTMLPurifier_Config
     {
         $config = HTMLPurifier_Config::createDefault();
         $config->set('HTML.DefinitionID', 'ilias forum post');
-        $config->set('HTML.DefinitionRev', 2);
+        $config->set('HTML.DefinitionRev', 3);
         $config->set('Cache.SerializerPath', ilHtmlPurifierAbstractLibWrapper::_getCacheDirectory());
         $config->set('HTML.Doctype', 'XHTML 1.0 Strict');
 
@@ -37,6 +37,16 @@ protected function getPurifierConfigInstance(): HTMLPurifier_Config
         $config->set('HTML.AllowedElements', $this->removeUnsupportedElements($tags));
         $config->set('HTML.ForbiddenAttributes', 'div@style');
 
+        $config->autoFinalize = false;
+        $config->set(
+            'URI.AllowedSchemes',
+            array_merge(
+                $config->get('URI.AllowedSchemes'),
+                ['data' => true]
+            )
+        );
+        $config->autoFinalize = true;
+
         if (($def = $config->maybeGetRawHTMLDefinition()) !== null) {
             $def->addAttribute('img', 'data-id', 'Number');
             $def->addAttribute('a', 'target', 'Enum#_blank,_self,_target,_top');