Add OG support for Twitter, Linkedin and Facebook

The current preview looks like this and needs an image: https://share.riseup.net/#MKKKaVIt6dJG-5nDvwuaEQ

I'm not sure the exact size to use, but this could work: https://share.riseup.net/#0gABIrtrdOO4nwjAKoGJXA