Allow approvals to be disabled for node types

Problem/Motivation

Currently, the approver workflow is applied to all node types. We should be able to disable approvals per content type.

Steps to reproduce

None.

Proposed resolution

Introduce a "disallowed_content_types" setting and enforce it.

Note that we will "disallow" since that respects the current assumption that "all types are allowed" and will not require an update hook.

Remaining tasks

Introduce a "disallowed_content_types" setting and enforce it.
Update tests

User interface changes

Adds a settings page

API changes

None -- but this should be done after 🐛 Service decorator must use the "inner" pattern Fixed and 📌 Use inner decorator for ApproverStateTransitionValidation Needs review for clarity.

Data model changes

Adds settings schema for "disallowed_content_types"