rewrite API

Current API implementation is becoming hard to extend and maintain. Here are base points that should be considered when rewriting API:

• all input/output should be entity based json (except when passing file content)
• serialization should be done automatically using Jackson
• patch/post/delete requests should handle properly simultaneous modifications of the objects (https://www.mscharhag.com/api-design/rest-concurrent-updates)
• when accessing collections use Pageable interface
• use JPA validation for input parameters