refactored Response Header and Body classes after finding some bugs documenting
updating UsersController with new syntax from Response::Body and Response::Header
documenting response messages for UserControllerDocumentation
added leftover Response::Body and Response::Header classes to project
adjusted Applicatio's Controlelrs with new syntax from Response::Body and Response::Header
fixed uniqueness of Checkpoint.sent_at
removed top level constant call
created swagger documentation for Checkpoint and CheckpointsController
included response_message codes for Gadget and User Controller documentations