A gallery of images (Lexical type "gallery"): attachments shown
together, in a grid.
A gallery is valid only under the root, and holds only
Kotoba.Nodes.Attachment nodes, in their order. The editor makes a
gallery of the images uploaded together, and of the images that the
person groups; it makes one image of a gallery with one image.
In HTML, a gallery is a div.kotoba-gallery with the figure of each
attachment. With the :untrusted policy, it is a p with the names of
the files. Its text is the names, one a line, and its Markdown the
images, one a line.