* Add a banner mixin, helping future releases and maintenance

* Update scss/bootstrap-reboot.scss

Co-authored-by: Julien Déramond <julien.deramond@orange.com>

* Apply suggestions from code review

Co-authored-by: Julien Déramond <julien.deramond@orange.com>
Co-authored-by: Mark Otto <markd.otto@gmail.com>