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/i18n/English/Account_Shop.pm

@@ -44,6 +44,11 @@ our $I18N = {
         lastUpdated => 1119068809,
     },
 
+    'userIsVendor' => {
+        message     => q{A boolean that is true if the current user is a vendor.},
+        lastUpdated => 1263833421,
+    },
+
     'common account variables' => {
         message     => q{Common Account, Shop Plugin, Template Variables.},
         lastUpdated => 1230867169,