Allow the app to be configured

Today we build the application with its configuration hardcoded via pug.

This has a huge downside: we have to build the app for each configuration and each time we change it. This means having node installed, download half the internet for dependencies and execute the build itself.

Being able to have a configuration living outside the code of the distributed app would make it much easier to maintain. I mean, do you compile your email client each time your change a parameter ?

It's not clear to me how we should handle that but I guess we could serve the config.json along with the index.js and the first thing the application would do is recovering this configuration file then use its content for the rest of the execution flow.

With this, I could make one building step for all the gitlab CI. Today we need 11 just for the sib-app and it's gonna increase with time. The more we will put sib-apps in production the more time it will take to deploy.