Finalize Onboarding welcome step

[gregorylegarec]

Thanks @CPatchane to have a review.

This PR is a little bit huge but it was hard to split it in several parts afterwhile. So sorry about that.

It essentially does :

• Fix somme little typos or issues server-side detected during integration
• Add a save() method to steps to be able to save content each time a step is completed. It also add related docs.
• Finally, it fixes and simplifies routes management :
  • Synchronizes server and client (this last one was too complex)
  • Makes serveral changes into Application
  • User is no more redirected to login page when an username exists in DB
  • In client Application every register route is now handled by a single method that shows onboarding's current step.

    Technical debt :

To save its data, the welcome step uses fetch(), but no polyfill is provided. This can be done with webpack. Also, I encountered difficulties to test this method, so no test is provided at this time.

[CPatchane]

Great job! Thanks @gregorylegarec 🎉