Refactor radio and checkbox group spacing

[jamesmockett]

Spacing on individual radio and checkbox components, and when part of a group is inconsistent. The components have a minimum height which implicitly creates spacing between them when stacked in a group (unless the label wraps), but also explicitly add an external margin when there is supporting text. This means the spacing behaves inconsistently and there is sometimes additional space around a group or individual component, or sometimes space missing when the label wraps.

Fixing this enables #895