use Entity API base classes; remove custom route definitions

Problem/Motivation

This module defines custom routes (such as restui.list, restui.edit) and plain form classes.

However, the Entity API provides better base classes for all of this functionality, which would reduce the amount of code in this module.

- ConfigEntityListBuilder
- EntityForm

Steps to reproduce

Proposed resolution

Implement hook_entity_type_build() to add the list controller & link templates to the entity type definition
Convert the classes to new parent classes

Remaining tasks

User interface changes

API changes

Data model changes