Cleanup needed

[davb]

Before we work further on this tool, I would like to do the following:

• use async/await instead of ugly Promises. In most of the code promises are not chained properly. Require Node 8+ in package.json to enable this.
• add code linting with eslint
• fix indentation: some files use tabs, other spaces. All should be set to two spaces
• rename the middleware folder to services. Middlewares are functions that take (req, res, next) and respond to requests. Functions that deal with data should be called services or model.
• move all constants to constants.js, especially environment variables; throw errors when required constants are blank

[davb]

@tjeanneau @lourou what do you think?