Add encapsulation blocks sonata_nav and sonata_breadcrumb

I've added two encapsulating blocks around navigation and breadcrumb to allow people do not rendering them when they are extending this layout.

Blocks are:

```twig
{% block sonata_nav %}
...
{% endblock sonata_nav %}
```

and

```twig
{% block sonata_breadcrumb %}
...
{% endblock sonata_breadcrumb %}
```