Fix notebook thumbnailer

[magjac]

This is explained here:

Feature request: Supply a fixed thumbnail

The thumbnail daemon selects the first image, SVG or Canvas element that is at least 320px wide and 200px tall. If there is no such element, then the top part of the notebook is used (immediately below the title). When we improve our publishing flow, we plan on offering greater control over the thumbnail, as well as the title and slug.