Added assets to config to allow js/css customization

Fixes https://github.com/sonata-project/sandbox/issues/357
Fixes https://github.com/sonata-project/sandbox/issues/358
Do we need to install any other package like bootstrap in order to have in a fresh install

create-project sonata-project/sandbox:2.3.x-dev

the missing resource as shown in the firebug:
http://www.sonata.loc:82/bundles/sonataadmin/vendor/jquery/dist/jquery.min.js
http://www.sonata.loc:82/bundles/sonataadmin/vendor/bootstrap/dist/js/bootstrap.min.js (2 times)
http://www.sonata.loc:82/bundles/sonataadmin/vendor/bootstrap/dist/js/bootstrap.min.js
thank you