Avoid security vulnerability with attribute target='_blank'

Using this attribute unaccompanied by rel='noreferrer noopener' is a
severe security vulnerability according to
https://mathiasbynens.github.io/rel-noopener/.
The target='_blank' attributes should be accompanied with
rel='noreferrer noopener'.

src/KeyboardShortcutsDialog.js

@@ -75,7 +75,7 @@ class KeyboardShortcutsDialog extends React.Component {
               </TableBody>
             </Table>
             <DialogContentText variant="body1">
-              For keyboard shortcuts in the text editor, please visit <a href="https://github.com/ajaxorg/ace/wiki/Default-Keyboard-Shortcuts" target="_blank">Ace Default Keyboard Shortcuts</a>
+              For keyboard shortcuts in the text editor, please visit <a href="https://github.com/ajaxorg/ace/wiki/Default-Keyboard-Shortcuts" target="_blank" rel="noreferrer noopener">Ace Default Keyboard Shortcuts</a>
             </DialogContentText>
           </DialogContent>
         </Dialog>