Delete activities

I think this is more involved. Since Jekyll is mostly state-less, we don't know which posts have been removed, unless we're building the site on top of a previous build, and then we only have access to a list of files to be removed, during the :cleanup hook.

This happens on Sutty, but not on other CIs, and even then we can't always guarantee it (it hasn't happened, but we have the option to rebuild the site after removing the previous build).

Removed posts also break backlinks, so it may be better to mark the post as removed, but keep the file, instead of removing the file and all traces of its previous existence. This would prevent 404s, which could be replaced by 410 Gone messages, that could be more informative (to whom?).

It doesn't cover the case where a post with the same URL replaces it later. For instance, a person could create a post, publish it, later remove it and create a new page that replaces it. The URL is the same, but the activity is different.

Similar to the "Tombstone" conversation we had. Do we have a better name for them?