Remove idenitfier/data wrapper, take 2

[dafeder]

fixes [/issues/3450]

• All tests pass
• JSON metadata stored in the database is not wrapped with with [{"data":{},"identifier":""}]
• API GET requests return the same JSON format as before (the data is wrapped with [{"data":{},"identifier":""}])

QA Steps

• Post a dataset with referenced entities (distributions, publisher, keyword, etc)
• Make sure that JSON metadata stored in the database is not wrapped with [{"data":{},"identifier":""}]
• Run GET requests to get entities of various schemas. Make sure that JSON output has not changed compared to 2.x.