Added SURVEYURL inside replacement array list

For below given Bug raised, have made change in frontend_helper.php file for adding a SURVEYURL inside replacement array list to be used at the confirmation template. 

http://bugs.limesurvey.org/view.php?id=7663
Merge pull request #73 from ravindrakhokharia/patch-1

Fixed issue: Unable to use SURVEYURL in confirmation email