Validation-related changes (refs #7289)

* Change 'validate_on_enrich' to 'valid_after_enrich'
* Add 'validation_message' to search schema and JSON-LD context
Version 2.1.14
Merge branch 'develop' of github.com:dpla/platform into develop
Version 2.1.15
Merge branch 'release-v2.1.15' into develop