luci-base: add FULL_REQUEST_URI template property

luci-base: add FULL_REQUEST_URI template property

Introduce a new template property FULL_REQUEST_URI which returns the full
canonicalized request URL built from SCRIPT_NAME, PATH_INFO and QUERY_STRING.

This new property is safer to use compared to using the raw REQUEST_URI CGI
environment variable directly as this value is essentially untrusted user
input which may contain embedded escaped slashes, double forward slashes and
other oddities allowing XSS exploitation or request redirection.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>