Replace branch approach by monorepo

[lucleray]

• each lesson is a folder (each item in the next-site/learn sidebar is a folder, with the exception of getting started)
• update dependencies to latest
• add prettier to fix formatting conflicts
• it is not needed anymore to stop/start next
• add readme to the repository explaining how to clone/download the repo
• fix 'url' property is deprecated
• add name to unnamed components
• use functions instead of arrow functions

todo :

• fix this issue in E2-firebase-hn
• create PR with appropriate changes in next-site/learn

room for improvement:

• add cross-env for NODE_ENV=production node server.js
• replace webpack-bundle-analyzer by @zeit/next-bundle-analyzer
• lazy loading components lesson needs to be updated

fixes #26
fixes #24

@timneutkens
Which one do you prefer:

• the content of a folder is what is expected at the end of the lesson (e.g. 1-getting-started contains what you get when you're done with the Getting Started lesson)
• the content of a folder is what you should start with at the beginning of a lesson (e.g. 2-using-shared-components contains the files to start the Using shared components lesson)

[lucleray]

I'm closing this because there is a new repo for it here : https://github.com/zeit/next-learn-demo