Re #8476 - Implemented and tested `get_cert_root(id)`.
Re #8476 - Implemented and tested `get_cert_details(id)`.
Re #8476 - Added and tested `get_certificate(id)`.
Re #8476 - Fixed and tested filtering for GET /api/certificates
Re #8476 - Refactored the application; swagger is not used for code generation anymore, REST API is being implemented from scratch. Migration only, fixing tests.