luci-mod-status: fix potential XSS via specially crafted DNS names

When an upstream NS returns PTR domain names containing HTML, it is
added verbatim to the connection status table.

Prevent this issue by HTML escaping any values in the source and
destination columns.

Fixes: CVE-2021-32019
Signed-off-by: Jo-Philipp Wich <jo@mein.io>