Clean up ENV var setup

Removed the redundancy of having files you have to copy and then paste config vars into. Everything now refers to the ENV vars, including Dev and Test DB vars (Prod and Staging deployments generate DB configs on the fly). Also added a var for the Rails secret token, which shouldn't be in a public repo. Updated the README instructions as well.

This change will require all Devs to double check and update their ENV vars as required.