Adjust how the "action" param is submitted in order list page

Before this change, the "download as CSV" button wasn't working as there
were two hidden "action" inputs and only the value of the "change order
status" button was being picked up.

This change uses button name attributes to determine which button was
submitted.

oscar/templates/oscar/dashboard/orders/order_list.html

                     <div class="pull-right">
                         <div class="form-inline">
                             <label>{% trans "Download selected orders as a CSV" %}</label>
-                            <input type="hidden" name="action" value="download_selected_orders" />
-                            <button type="submit" class="btn btn-primary" name="download_selected">{% trans "Download" %}</button>
+                            <button type="submit" class="btn btn-primary" name="action" value="download_selected_orders">{% trans "Download" %}</button>
                         </div>
                     </div>
                 </caption>
                                 
                             </div>
                         </div>
-                        
-                        <input type="hidden" value="change_order_statuses" name="action" />
-                        <input type="submit" value="{% trans "Change status" %}" class="btn btn-primary" />
+                        <button type="submit" name="action" value="change_order_statuses" class="btn btn-primary">{% trans "Change status" %}</button>
                     {% else %}
                         {% trans "This order can't have its status changed." %}
                     {% endif %}