Display external images and original links on thanks

Previously, external images were mucked up and links were routed through
l.php, which requires a client sign in. If a client were not signed in when
creating a ticket. The links would not work correctly.
Merge pull request #998 from greezybacon/issue/994

Display external images and original links on thanks

Reviewed-By: Peter Rotich <peter@osticket.com>