templates/submission: add button to copy the message ID to clipboard

Add an icon alongside the message ID to copy the message ID to the
clipboard, to ease integration with eg b4.

Signed-off-by: Ross Burton <ross.burton@arm.com>

patchwork/templates/patchwork/submission.html

@@ -28,6 +28,7 @@ <h1>{{ submission.name }}</h1>
 {% if submission.list_archive_url %}
       (<a href="{{ submission.list_archive_url }}">mailing list archive</a>)
 {% endif %}
+      <span class="btn-link btn-copy glyphicon glyphicon-copy" data-clipboard-text="{{ submission.url_msgid }}" title="Copy to Clipboard"></span>
     </td>
   </tr>
 {% if submission.state %}