Site Notice: common_purify instead of HTMLPurifier

So that we can use our custom settings (e.g: extra URI schemes) in the
site notice textbox.

Ref. #170

actions/sitenoticeadminpanel.php

@@ -110,9 +110,7 @@ class SitenoticeadminpanelAction extends AdminPanelAction
         }
 
         // scrub HTML input
-        require_once INSTALLDIR.'/extlib/HTMLPurifier/HTMLPurifier.auto.php';
-        $purifier = new HTMLPurifier();
-        $siteNotice = $purifier->purify($siteNotice);
+        $siteNotice = common_purify($siteNotice);
     }
 }