Problem/Motivation
Follow-up to #2725533: Add experimental content_moderation module. Add constraint that enforces unique content_entity_type_id, content_entity_id and content_entity_revision_id.
Proposed resolution
Add constraint and discuss whether we should enforce on the data level.
Remaining tasks
User interface changes
None
API changes
New constraint
Data model changes
New constraint and perhaps change index on table to enforce at the data level