Add support for Twitter Cards and Open Grapf metadata

When posting links to pony mail on social networks it's not present well (especially perm link which is ugly). Change this by adding metadata in twitter and opengraph format. This will allow better SEO too.

See:
* http://ogp.me/
* https://developer.twitter.com/en/docs/tweets/optimize-with-cards/overview/abouts-cards.html

Start by locating the page or template that generates Pony Mail links and permalinks, then review the linked Open Graph and Twitter Cards specifications. Add the requested metadata for those pages and verify that shared links expose useful titles, descriptions, images, and permalinks.

Written by the indexing model from the issue text.