Better image handling 2

[cloverich]

In #232 i'd hoped to do this, but instead added very basic height constraints to images, removed some buggy captioning features, and deferred the fancier image handling, which this can now be.

Images by default have:

• have caption support
• A small set of pre-defined layouts for grouping images
• A toggle to change them (e.g. top right)

Layouts could be something like:

But the key UX is being able to drag and drop groups of images and having something simple and useful happen.