Make open graph and other meta data configurable in default template

When you create or edit a page, you should be able to set a feature image.

Example: Set a feature image on the team page, so when you share you see something like the group photo.

Todo: Also see what other meta data could be configurable.

mentioned in merge request !1366 (merged)

mentioned in issue #518

Should also include twitter card meta data.

https://dev.twitter.com/cards/markup