This is a fix for ticket #7670. Some hosting situations don't
allow write access to the system temp path. tiny_mce_gzip.php is currently
using sys_get_temp_dir() by default, and not using a local silverstripe-cache
folder that may exist in the SilverStripe project.
This change moves the getTempFolder() function into a common file, and
includes that in core/Core.php, as well as thirdparty/tinymce/tiny_mce_gzip.php
so both locations share the same code to work out the temp path.
