Add deeper UI integration for content_moderation

Problem/Motivation

It currently works to save the moderation_state when adding it as a site setting value. But the UI does not respect this since the content_moderation module alters the forms and loads the status by itself, which of course does not respect the override.

Steps to reproduce

- Add moderation_state and status as overrideable fields
- Create an override where moderation state gets changed
- Go back to the override form, it now displays the wrong value
- Also even though reverting works, the override status will always remain overriden since the moderation_state and status field always require a value and the logic checks for isEmpty

Proposed resolution

- Add submodule for plug&play integration (implement the overrideable fields hook)
- Alter form to fix values
- Make override status alterable and add custom logic

API changes

- Add new hook to alter the has_override_for_site behavior