Add opengraph meta tags for better social media experience

[valioDOTch]

The current system does not provide any opengraph meta tags. Thus sharing a link to an adhocracy installation usually results in a quirky assumption of what should be displayed in the link preview by Facebook e.g. see https://developers.facebook.com/tools/debug/og/object/?q=https%3A%2F%2Funsere.skuba.ch%2Flogin%3Fcame_from%3Dhttps%253A%252F%252Funsere.skuba.ch%252F