Bug 1759367: Cherry-pick for Allow CSS comments in customised skins.

Cherry pick of Bug 1369830 for the upgrade of html purifier to 4.10.0 


As we add a new configuration option to HTML Purifier,
we have to update HTMLPurifier/ConfigSchema/schema.ser
 1. Download and unzip HTMLPurifier package
 2. Copy Filter.ExtractStyleBlocks.PreserveCSS.txt
from mahara to the unzip folder
 3. Run php maintenance/generate-schema-cache.php in
unzup folder
 4. Copy over lib/HTMLPurifier/ConfigSchema/schema.ser
from unzip folder to mahara

behatnotneeded

Change-Id: I42618f0998425d20f7732843af8a700260c8d96e

htdocs/lib/htmlpurifier/HTMLPurifier/Filter/ExtractStyleBlocks.php

@@ -121,6 +121,10 @@ class HTMLPurifier_Filter_ExtractStyleBlocks extends HTMLPurifier_Filter
      */
     public function cleanCSS($css, $config, $context)
     {
+        if ($config->get('Filter.ExtractStyleBlocks.PreserveCSS')) {
+            $this->_tidy->set_cfg('preserve_css', true);
+        }
+
         // prepare scope
         $scope = $config->get('Filter.ExtractStyleBlocks.Scope');
         if ($scope !== null) {