Update add_block.html.twig

At the original add_block.html.twig file it was used ROLE_SUPER_ADMIN, but in the standard_layout.html.twig it is used the ROLE_SONATA_ADMIN.

I suppose that the ROLE_SONATA_ADMIN should be the global role.

At my application broke the presentation when the user has the ROLE_SONATA_ADMIN but not the ROLE_SUPER_ADMIN.
Merge pull request #2092 from jglete/patch-1

Update add_block.html.twig