Add support for entity types without bundles

Problem/Motivation

The entity_type_behaviors module currently only has a UI for content entity types that have bundles.

Actually entity_type_behaviors already works with entity types that don't have bundles when a entity_type_behaviors.entity_type_bundle.ENTITY_ID.ENTITY_ID.yml config is added manually.

Tasks

Add a UI for content entity types that don't have bundles.