Refactor Code to Separate Concerns

[BethanyG]

The code needs to be refactored into a "proper" Express app. The three current files need to be broken down into the "usual" tree. Something like what's generated here:

https://expressjs.com/en/starter/generator.html

This is so changes/additions to routes don't conflict or break changes made to messages or message actions. Endpoints need to have their own files and their own associated assets. Assets need to be separated from actions, and the incoming message parsing logic needs to be in it's own file. Configuration files need to be made for Staging and Production respectively.

[BethanyG]

First pass of this is now in #15. It will probably needs some additional work, but is more or less chunked out.