FIX allowed pagetypes displaying incorrectly when switching subsite

This patch depends on an update to the CMS module that provides this
extension point. The code is inert when matched with existing CMS
versions.

_config/extensions.yml

@@ -50,6 +50,7 @@ SilverStripe\Admin\SecurityAdmin:
 
 SilverStripe\CMS\Controllers\CMSMain:
   extensions:
+    - SilverStripe\Subsites\Extensions\HintsCacheKeyExtension
     - SilverStripe\Subsites\Extensions\SubsiteMenuExtension
 
 SilverStripe\CMS\Controllers\CMSPagesController:

src/Extensions/HintsCacheKeyExtension.php

@@ -0,0 +1,22 @@
+<?php
+
+namespace SilverStripe\Subsites\Extensions;
+
+use SilverStripe\CMS\Controllers\CMSMain;
+use SilverStripe\Core\Extension;
+use SilverStripe\Subsites\State\SubsiteState;
+
+/**
+ * This extension adds the current Subsite ID as an additional factor to the Hints Cßache Key, which is used to cache
+ * the Site Tree Hints (which include allowed pagetypes).
+ *
+ * @package SilverStripe\Subsites\Extensions
+ * @see CMSMain::generateHintsCacheKey()
+ */
+class HintsCacheKeyExtension extends Extension
+{
+    public function updateHintsCacheKey(&$baseKey)
+    {
+        $baseKey .= '_Subsite:' . SubsiteState::singleton()->getSubsiteId();
+    }
+}