[WIP] Twig entity embed

Here's the corresponding d.o issue. https://drupal.org/node/2279257

It is built on 8.x-1.x right now instead of the embed-display branch. This PR will probably stay Work-In-Progress until it uses  EntityEmbedDisplay.
Over in https://github.com/drupal-media/entity_embed/pull/32 I got rid of a direct entity_view() with dependency injection. I will try the same here.