MAGETWO-35182: The link of "URL" type breaks "edit Downloadable" Backend page

Author: Natalia Momotenko

app/code/Magento/Downloadable/view/adminhtml/templates/product/edit/downloadable/links.phtml

@@ -57,7 +57,7 @@
                         </thead>
                         <tfoot>
                             <tr>
-                                <td colspan="8"><?php echo $block->getAddButtonHtml() ?></td>
+                                <td class="col-actions-add" colspan="8"><?php echo $block->getAddButtonHtml() ?></td>
                             </tr>
                         </tfoot>
                         <tbody id="link_items_body">