Fix problems with the Sales tab in the Shop Account plugin.

Only display the Sales tab to vendors, and prevent www_viewSales from erroring out if the
user is not a vendor.  New template, template variable, i18n.

lib/WebGUI/Help/Account_Shop.pm

@@ -18,6 +18,7 @@ our $HELP = {
         variables => [
             { name => "manage_purchases_url", },
             { name => "managePurchasesIsActive", },
+            { name => "userIsVendor", },
         ],
         related => [ ],
     },