Move routes to ./routes.js & convert last few definitions to app_route

I've been checking our the structure of other Express/Mongoose projects and found https://github.com/jedireza/drywall. I like how they structure their app, so I'll be adding a few PRs that reflect some of the smaller things.

`app.js` is our main entrypoint. Right now, we have over 100 lines of route definitions. Might as well move it to it's own file.