refactor: Move boilerplate functions into Express 'res' object.

This obsoletes the boilerplate wrapper, simplifies request handlers, and makes the boilerplate API feel more natural.
refactor: Consolidate repeated template data code in reusable variables.
refactor: Implement push_error_object boilerplate and use it.

This removes repetitive conditionals inside request handlers.
refactor: Use push_error_object in login route error handling logic.