Allow to set the default language of the new users

[marien-probesys]

Problem

At Probesys, we need that the default language to be French. However, it's always English by default.

Solution

Allow to define a default locale in the global configuration.
Allow to set the user's locale when creating it in the admin and with the CLI (related to #490)

Specification

• Add an environment variable APP_DEFAULT_LOCALE and use it instead of the \App\Utils\Locales::DEFAULT_LOCALE where needed
• Refactor the User new/edit forms with the Symfony Form component (related to Refactor forms with the Symfony Form component #551)
• Add a locale field in the "new user" form
• Add a locale option to the CreateCommand