Upgrade frontend build system to npm & webpack

This Merge request is Teklia's work on upgrading the frontend build system to use npm & webpack.

• There are no vendored libraries anymore (except color-thief due to an upgrade we need to discuss)
• django-compressor is removed in favor of django-webpack-loader
• Four chunks (Js & CSS) are generated by the new system:
  1. vendor has all the 3rd parties libraries
  2. main contains all the Vanilla JS source code used on most pages
  3. baseline_editor contains the Vanilla used on /baseline-editor/
  4. editor contains the Vue.js application code
• the app/Dockerfile now builds the frontend in a nodejs image, then the existing python based image use the resulting build
• as we use webpack, we need to explicitly export the functions or classes used by top-level javascript (from the Django templates). This allow developers to explicitly set boundaries between HTML & JS code
• Two CI tasks were added:
  1. build the frontend, using production setting
  2. build the docker image
• as little JS code as possible was modified:
  • adding export where needed
  • replacing the document.onload calls by function encapsulation (prefixed by boot) to control clearly each temlate's initialization
• of course we tested as much as possible with our current knowledge of the application.

To build the frontend:

cd front
npm install # run once
npm run build

You can also test with a simple docker-compose up --build