Correctly implement OpenGraph tags

Just defining og:site_name indicates that we support the
OpenGraph protocol, but then we didn't set the required
properties.
There's no concept of multiple langauges here, the OpenGraph protocl does not provide any means to request content in multiple languages, it knows to list available alternate locales via og:locale:alternate, but dosn't even specify how a client could retrieve them. There are no means to associate a language to a key. So the only sane choice for us is an English message here.