RPC: Fixed JsonSerializer implementation

- Do not serialize userExtSources property of Candidate.
  Was set on "getDeserializationConfig()" which was
  probably typo and caused sending of unnecessary data with
  object (same data is in userExtSource and
  additionalUserExtSources properties).

- Fixed serialization of PerunException, since it contained
  unnecessary properties like: localizedMessage & stackTrace.

- Applied Java source formating, optimized imports.