Add plain list element as a helper

@moneyadviceservice/frontend 

A plain list to go along with unstyled-list as per @ourmaninamsterdam advice.
I'm getting confused over the differences between and unstyled-list and a plain-list
unstyled-list has no bullets and no padding or margin
plain-list has no bullets

Is no-bullets (or no-bullets-list) a clearer name than plain-list?