Zombie process when killing `make dev`

STR:

• Launch make dev
• When the server is being built, kill make dev with Ctrl + C (if you do it when the client is being built, this won't work)

Observed:

• You just lost control over the client build script, and cannot kill them with Ctrl+C

A first analysis has shown the main problem is a bug in npm (used to run the server build), even though all scripts miss set -e to also fail correctly. When using npm run something inside a shell script, it won't return the error code of the script behind the command something (using scripty or not - same analysis with concurrently).

An easy fix would be to replace npm by yarn in all scripts (and add yard to the dev dependencies)