dev_tools: Add invalid confirmation error page to devtools.

This just makes it a bit easier and more self-service to get to this
category of error pages.

templates/zerver/dev_tools.html

@@ -59,6 +59,11 @@
                 <td>None needed</td>
                 <td>Error 5xx page served by Django</td>
             </tr>
+            <tr>
+                <td><a href="/accounts/do_confirm/invalid">/accounts/do_confirm/invalid</a></td>
+                <td>None needed</td>
+                <td>Invalid confirmation link page</td>
+            </tr>
         </tbody>
     </table>
     <p>Development-specific management commands live in <code>zilencer/management/commands</code>.  Highlights include: