enhanced plugin support (#634)

- refactored admin controller and templates
- plugin support for entity actions
- changed column mail to email in customer table
- refactored theme events

templates/invoice/renderer/freelancer.html.twig

@@ -53,7 +53,7 @@
                 </tr>
                 {% if model.query.project is not empty and model.query.project.orderNumber is not empty %}
                     <tr>
-                        <th>{{ 'label.order_number'|trans }}</th>
+                        <th>{{ 'label.orderNumber'|trans }}</th>
                         <td contenteditable="true">
                             {{ model.query.project.orderNumber }}
                         </td>