Add social tags & special message on share links

[paolodamico]

It stands to reason that Debin messages will be shared in social platforms, particularly things like Slack or Discord. We should add a nice og:image to display there. Further, when sharing a specific message (i.e. https://debin.io/#/m[...]), we should change the description to something along the lines of "Someone is sharing a message with you through a web3 decentralized network".