homepage: add youtube link and icon
homepage: add youtube link and icon

ref https://trello.com/c/zpQNAp8q/52-you-tube-in-djangocon-site

djangocon/2017.djangocon.eu,djangocon/2017.djangocon.eu,djangocon/2017.djangocon.eu,djangocon/2017.djangocon.eu,djangocon/2017.djangocon.eu