Allow editorial control of image presence within Prismic

[davidpmccormick]

When migrating Prismic content we removed the standalone and supporting labels since they would stop working in Slice Machine land. We also decided not to re-implement them using an extra field on the type (as we did for e.g. quotes) because the way they were implemented meant there was no good way of knowing if they were going to have the desired effect on the image when it was rendered on the frontend (based on the image aspect-ratio and, crucially, the user's browser).

The photography editors still have a desire to give different images varying degrees of presence within the body of a page so we should find a way to accommodate this.

It might be a case of treating landscape and portrait images differently so that they proportionally take up similar amounts of space on a page. Potentially with an option to limit any image to a maximum number of grid columns.

Propsal from photography: ImageSize Proposal.pdf