Neaten "void invoices" button

Only want to show this once - not once per invoice.
It's not something most people will need to use so it doesn't need to
be a button. Restyle it to be a link

pinaxcon/templates/dashboard.html

@@ -124,8 +124,8 @@
                               {% endif %}
                                 <a href="{% url "invoice" invoice.id %}" >Invoice {{ invoice.id }}</a>
                                 - ${{ invoice.value }} ({{ invoice.get_status_display }})
-                                <button class="btn btn-lg btn-default" type="button" id="toggle-void-invoices" href="" onclick="toggleVoidInvoices();">Show void invoices</button>
-                              </li>
+                                </li>
+                                <a id="toggle-void-invoices" href="" onclick="toggleVoidInvoices();">Show void invoices</a>
                             {% endfor %}
                           </ul>
                         </div>