Fix Opengraph metadata and improve teams page

• og:imagem was using relative url (which facebook does not support)
• team page does not provide a "page_title" metadata, which is used to fill out og:image meta tag.

I've made a MR to fix both. I'm using suggested image from last summit as our og:image :)

Plus: moved team metadata to a datafile in source/_data/team_members.yml source/_data/team.yml

Will solve #466 (closed).