[bp/r52126][FIX]Convert the array of mail_user into a string of emails separated by comma, and expose the values to the smarty tpl. Allows to fix bug5369

git-svn-id: file:///var/local/code.tiki.org/tikisvn/branches/12.x@52127 b456876b-0849-0410-b77d-98878d47e9d5