Add mailcatcher, deactivate Celery and make settings more flexible

• Improve email settings to allow flexible configurations in each environment,
• Add mailcatcher to handle the emails sent by edxapp in fun-platform,
• Deactivate Celery to simplify development in fun-platform,
• Move environment variables to environment files,
• Allow formatting settings passed as environment variables and apply it for celery, mailcatcher and the database configurations,
• Fix "update_assets" in development Dockerfile to avoid the uninstalling/reinstalling that Open edX does by default.