multiEdit: support validate on custom action

If requireValidation: true is set for a custom action, then only
perform action if multiEdit's form fields are valid. This does not
apply to clicking on existing rules' action buttons.

Syntax:

multiEdit: {
  fields: {
    testField: {
      custom: {
        requireValidation: true,
        ...
      },
      ...
    },
    ...
  }
}

ui/scripts/ui/widgets/multiEdit.js

@@ -804,6 +804,9 @@
         $('<div>').addClass('button add-vm custom-action')
           .html(_l(field.custom.buttonLabel))
           .click(function() {
+            if (field.custom.requireValidation &&
+                !$multiForm.valid()) return false;
+            
             var formData = getMultiData($multi);
             
             field.custom.action({
@@ -816,6 +819,8 @@
                 }
               }
             });
+
+            return false;
           }).appendTo($td);
       } else if (field.addButton) {
         $addVM = $('<div>').addClass('button add-vm').html(