Refactor/http api

[zrgt]

• Refactored handling of get_response_type():
  To remove get_response_type() in the beginning of all endpoint funcs I have integrate it in handle_request() and added an arg for response type in all endpoint funcs.
• Refactored _get_submodel_or_nested_submodel_element():
  It has only url_args now as an argument and returns the se or a submodel.
  Before it had a submodel and id_shorts in arguments, so in all funcs which used it a submodel and id_short_path were explored with separate funcs.

Depends on #54