Move Twig Text extension to core bundle

This is related to PR https://github.com/sonata-project/SonataCoreBundle/pull/33

As the sandbox now uses new kernel notions, `SonataAdminBundle` is not loaded in `FrontKernel` and this Twig Text extension was loaded in `SonataAdminBundle`.
Problem is that some front-end bundles (like `ecommerce` bundles and `SonataNewsBundle`) uses `truncate()` method that needs Twig Text extension to be loaded.

So I've moved Twig Text extension to `SonataCoreBundle` as both front & admin bundles will need it (and this need is shared between multiple bundles).