Connect FrontEnd to BackEnd Correctly

[Jooms]

From #101

1. Set up environment variables for the frontend to point to the correct backend. (Localhost when dev, EnvVar when prod)
2. Configure CORS correctly (from EnvVar) when deploying backend.

We should be able to use a runtime-injected value for what backend a frontend should be talking to, and what frontend a backend should listen to.

[MMurtey]

See the branch register_backend_connect. This works in dev, anyway. @prestonmasseyblake, it turned out not to be a CORS issue (it's already set to be permissive), but the backend not explicitly "listening" on port 3000. On that branch, registration now correctly posts data to the database and returns a simple JSON response in the form {"status": true/false, "message": details about operation results}. See also #120