Sharing optimisations

[slomo]

here are a few proposal what could be done better/different with our sharing tags. I didn't think them a 100% through yet.

• Instead of using the original URL forog.image we could provide a reasonably cropped version because it would load faster and we are getting warnings from facebook that our images aren't served fast enough. Also they seem to have an 8m limit, which we in theory could break.
• Drop twitter specifc tags. Twitter seems to support open_graph, so why even use two kind of tags.