update: switch to config.json as main config file

Switching the config file from src/includes/variables.pug to config.json

@Marjolaine please note that you need to create a config.json file containing something like:

{
  "serv": "http://127.0.0.1:8000",
  "connected": "http://localhost:3000"
}

For the application to work. This one is not versioned because:

• It's a config file
• It will be overriden by the autodeploy

Can you test it and confirm if that works ?