[1.5.x] Prevented arbitrary file inclusion with {% ssi %} tag and relative paths.

Thanks Rainer Koirikivi for the report and draft patch.

This is a security fix; disclosure to follow shortly.

Backport of 7fe5b656c9 from master
[1.5.x] Added 1.4.7/1.5.3 release notes

Backport of baec6a26dd from master