[DI-2712] Some minor improvements

[denys-chura]

• Add localhost and 0.0.0.0 to *_ALLOWED_HOSTS;
• Add *_DJANGO_ADMIN_URL and ADMIN_URL to be able to change admin url;
• Improve readme;
• Fix ConfirmResetPasswordSerializer.signature field length validation;
• Update CELERY_TIMEZONE to be set from *_TIME_ZONE environment variable instead of importing django.py settings file.