Add blocks inside the table and th tags in base_list.html.twig

The additional blogs allow adding attributes and classes to these tags
without needing to copy a much larger section of the template.

Some javascript libraries (FooTable.js for example) use data attributes to set
properties of the columns or table. This can easily be done by overriding
the new blocks.
Merge branch 'Update-base_list' of https://github.com/JDruery/SonataAdminBundle into Update-base_list