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
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