Add 'Save As' feature for dashboards (#1669)

* Add 'Save As' feature for dashboards

* Address code review comments

superset/assets/javascripts/components/ModalTrigger.jsx

@@ -7,6 +7,7 @@ const propTypes = {
   triggerNode: PropTypes.node.isRequired,
   modalTitle: PropTypes.node.isRequired,
   modalBody: PropTypes.node,  // not required because it can be generated by beforeOpen
+  modalFooter: PropTypes.node,
   beforeOpen: PropTypes.func,
   onExit: PropTypes.func,
   isButton: PropTypes.bool,
@@ -57,6 +58,11 @@ export default class ModalTrigger extends React.Component {
         <Modal.Body>
           {this.props.modalBody}
         </Modal.Body>
+        {this.props.modalFooter &&
+          <Modal.Footer>
+            {this.props.modalFooter}
+          </Modal.Footer>
+        }
       </Modal>
     );
   }