Use entity validation API rather than form API

Problem/Motivation

A limitation of the current implementation is that it relies on form-level validation and so applies only in certain conditions. While issues like #2598864: Validation for overview menu form and #2598866: Validation for node form attempt to mitigate this limitation by adding new form validation, this approach still misses any non-form menu item manipulation or creation, such as via REST.

Proposed resolution

Use the entity validation API, adding a validator to the entity type.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet