#108 Recreate project with create-react-app

[Nadeflore]

This changes the project structure to use the default react-create-app configuration.

Fix #108, fix #104, fix #101.

In order to make this work, the following changes has been made to the code:

• LESS was converted to SCSS;
• Replaced use of react router found in context (was private API) with use of withRouter (this is a consequence of updating to react-router 5);
• Absolute import are relative to the src folder, thus jsx folder content has been moved to src.

Bonus:

• Use optional chaining operator.

Remaining things to do:

• [x] Clean code to remove eslint warnings
• [x] Use production build with server

[Neraste]

Can we be sure that the following dependencies are still listed and used:

• Sprintf-JS;
• Post-CSS.

[Nadeflore]

sprintfs-js is a dependency of argparse. This explains why it still worked event with the dependency removed. I re-added it. about post-css, it is used by "autoprefixer" which is a depency added by react-scripts

[Neraste]

When testing the new workflow, songs in library cannot be expanded anymore.