dev: Reorganize and document the env files

[marien-probesys]

Context

While working on #916, I've realized that I could greatly improve the handling of environment variables in the application, especially to avoid the weird default: env var processor. The idea was to declare all the "production" environment variables in the .env file.

Then, I wasn't happy with the default values being those for the development environment. The .env variables are now suitable for production. The only exception is the APP_ENV variable which is set to dev in order to ease the workflow in development.

The development variables are now declared in a .env.dev file. This implied to copy some of them in the .env.test as well.

The consequences are:

1. production is not setup anymore with default (but not working) APP_SECRET, DATABASE_URL, MAILER_DSN, etc.
2. development variables must be overridden in a .env.dev.local file (.env.local doesn't work anymore to override e.g. DATABASE_URL)

I've also reorganized the environment variables in the env files, added documentation to explain how to manage these files, and put all the strings between strings.

This PR must be merged after the release of Bileto 0.14.0 to give me time to fix potential bugs due to these changes.

I want to test these changes with the prod Docker image too.

Related issue(s)

N/A

How to test manually

1. Run the application in dev / test / prod
2. Verify that it works

Reviewer checklist

• Code is manually tested
• Permissions / authorizations are verified
• New data can be imported
• Interface works on both mobiles and big screens
• Interface works in both light and dark modes
• Interface works on both Firefox and Chrome
• Accessibility has been tested
• Translations are synchronized
• Tests are up to date
• Copyright notices are up to date
• Documentation is up to date
• Pull request has been reviewed and approved