Meta headers should add a description, notably for talk details page

[clacote]

When tweeting a talk detail page like https://mixitconf.org/2018/les-dessous-d-un-embargo-de-securite, the Twitter card is empty (see https://twitter.com/mixitconf/status/992376164458139649):

In fact, there is no <meta name="description"> header.

It would be nice to have the "short description" of the talk as meta description.
Same for profile page, which could include the bio as meta description.