Add responses for user handlers

[itechdima]

Acceptance criteria:

• [ ] Add json responses for Create and Update;
• [ ] Add http status codes in response headers (200, 201, 500) (codes)

[itechdima]

Hints: server/user.go: Add convert function: func FromDomain(u pkg.User) User {return User{...}} server/server.go: use convert function and json marshal it server/server.go: w.Write(data)

for status codes: server/server.go: for successful update and delete w.WriteHeader(http.StatusOK), for successful create: 200, for error: 500