Layout blocks

Hi,

Here's the PR for the base_layout.html.twig from yesterday. You asked me to prefix the blocks with **sonata_**. If I prefix *html_tag*, *head_tag*, *head_bottom* and *body_tag* we will not be consisted with the rest of the blocks in the bundle like *page_top_bar*, *page_container* and so on. Any thoughts?

Cheers,
Stan