Styles for Flexible Sections still refer to old section names and have mild margin issues

Introduced in https://github.com/alphagov/frontend/pull/5534

Styles used by Image flexible section still refer to body-with-image, and the margins put in to Image / Document List / Share don't quite match the images they had when they were part of composite types.

A general audit of margins/styles of sections might be a good idea after the refactor.