Fix for #660

Campaign preview and campaign test send pulls the first entry in the RSS feed and substitutes its data in `[RSS_ENTRY_*]`

client/src/templates/helpers.js

@@ -683,6 +683,14 @@ export function getEditForm(owner, typeKey, prefix = '') {
                                 <Trans i18nKey="rssEntryImageUrl">RSS entry image URL</Trans>
                             </td>
                         </tr>
+                        <tr>
+                            <th scope="row">
+                                {tg('RSS_ENTRY_CUSTOM_TAGS')}
+                            </th>
+                            <td>
+                                <Trans>Mailtrain custom tags. The custom tags can be passed in via <code>mt:entries-json</code> element in RSS entry. The text contents of the elements is interpreted as JSON-formatted object..</Trans>
+                            </td>
+                        </tr>
                         </tbody>
                     </table>
                 </div>}