Add rel="noopener" to target="_blank" links

The noopener construct protects from some nasty clickjacking attacks. We
can apply them savely to all our links since we don't rely on the
previously used page.

Some more details: https://mathiasbynens.github.io/rel-noopener/

Signed-off-by: Sheogorath <sheogorath@shivering-isles.com>

public/views/codimd/body.ejs

@@ -113,7 +113,7 @@
             </div>
             <div class="modal-body" style="color:black;">
                 <h5></h5>
-                <a target="_blank" style="word-break: break-all;"></a>
+                <a target="_blank" rel="noopener" style="word-break: break-all;"></a>
             </div>
             <div class="modal-footer">
                 <button type="button" class="btn btn-default" data-dismiss="modal"><%= __('OK') %></button>