blog: atom feed and/or email subscription

[slint]

It would be nice to setup an Atom/RSS feed or an email subscription service. For the Atom feed there's already a plugin for Lektor, lektor-atom. For the email subscriptions, there are services that provide a decent number of "marketing" emails on their free tier like MailChimp and SendinBlue that may fit our needs.

[krzysztof]

👍 We could further hook it in on the frontpage, so instead of manually crafting the "What's New" box we would pull most recent blogpost title/short description.