Resolve "Manage database initialization with configuration files"

Closes #91, #40

Description

• Add a parameter to db initialization (i.e., input directory where json data are located)
• Move scheduler initialization from create_app to wsgi because init_db needs create_app but not scheduler
• Move test data in a new directory and update docker/run_app.py with this parameter

Also

• Manage different environments for backend (i.e., dev, qualif, prod, ci ?) #40

Remaining tasks

• Update documentation
• Add changelog

Out-of-scope (but TODO later)

• Add qualif/prod config files in deploy gitlab project