Entity embed config entity

d.o issue: https://www.drupal.org/node/2273361

@todo:
- Support for customizing button icons.
- ~~Remove entity type field from embed form.~~ [Done]
- Add CRUD tests.

`filter_format` was already upcasted and not `embed_button` (though I'm not sure why). Without this definition, it was treated as a string and not as an entity.