Add Open Graph Protocol (OGP) metadata to pages.

[sirocyl]

This allows for embedding content - small description, logo, title, perhaps some links - into socially shared URLs on platforms such as Discord, Slack, Skype, Twitter, Facebook; Telegram, Mastodon, Riot, Mattermost, NextCloud Talk, some XMPP implementations, among others.

This also improves SEO drastically - allowing for natural metadata embedding without "gaming" the system with keyword stuffing, other blackhat techniques.