Add unobtrusive undo feature when deleting a section

Right now there's no confirmation for deleting a section of a Gistbook. I like that, but I dislike how there's the risk of losing a lot of work. A solution to this would be a non-intrusive notification, a la Gmail, that lets you undo deletions.

Start by locating the section-deletion flow in the Gistbook codebase and inspect how deleted content is stored or removed. Then identify the existing notification or message UI, if any, and define done as deleting a section without confirmation while offering a non-intrusive way to restore it.

Written by the indexing model from the issue text.