All bookmark resource types should have the option to change author from Dual Field to Single Field

Some bookmark types (eg Book) have the option, when first bookmarking, to change the Author field from Dual Field to Single Field. For other bookmark types (eg Document), when first adding an Author field it can only be added as Dual Field; the option to switch to Single Field only appears after saving the bookmark and going back in to re-edit.

Documents (such as reports) very often have a corporate or organisational author, which for correct referencing needs to be entered as a single field - it's unnecessarily fiddly (and academics often forget) to have to save and go back in to get the Single Field option. All types of bookmark should have the option when first bookmarking to create the author as a Single Field.