Add configuration schema and validation for the model entities

Problem/Motivation

Configuration without a schema is configuration that fails at request time instead of at save time, and schema coverage is already a rule in this module. These entities are written by a generator, which makes malformed configuration more likely rather than less.

Proposed resolution

• Full schema for the structure, endpoint and conventions entities, including the shape settings, which are per-plugin and need a schema per shape.
• Constraint-based validation: a shape that cannot serve the field type it is pointed at, a structure referencing a missing bundle, a breadcrumb trail naming an unknown page role.
• Tests in the style of the existing configuration schema tests, plus a case per validation rule.
• Validation must be callable outside a form, because the generator and the drush commands write configuration directly.

Remaining tasks

Everything.