Refactor endpoints to separate game logic

[phblj]

An endpoint function (one annotated with @app.route) should only do the following:

• extract data from the request object (pulling from request.json and separating out the useful pieces)
• identify what's being requested by the client
• call a function or few functions to do what's requested
• combine the results of the functions into a response the client will understand

Clean up the existing endpoints so they're easier to read and offload data manipulation to helper functions.