Allow form group classes on radios and checkboxes

[kr8n3r]

What

Make it possible to toggle the govuk-form-group--error class on the form group for radios and checkboxes without having to pass an error message.

Why

Currently the error state for a conditionally revealed checkbox shows the 'error bar' against the revealed content only.

We want to move the error bar to the 'question' which will include the revealing checkboxes or radios themselves.

To do this we have in introduced a `formGroupClasses' option in the Nunjucks macro that allows a CSS class to be set directly on the form group.

Example of an error state set on the whole group

Trello ticket: https://trello.com/c/LggtR8s5/1455-make-it-possible-to-set-the-error-state-on-checkboxes-and-radios-without-passing-an-error-message

[NickColley]

I think if we want to add this potentially we want to make sure you can set formGroupClasses anywhere that formGroups are used? Or are these the only places?

Just thinking people will expect this to be consistent across components.

[kr8n3r]

@NickColley most likely, see #1042
scoping work for to the trello ticket. if we agree on the format of the parameter (this or one proposed in the linked issue) and we can roll that out to other components that group elements

[36degrees]

Are there other things that we can foresee users wanting to add form-groups in the future, like attributes? Just wondering if we should opt for formGroup: { classes: '' } instead.

I also wonder whether this is one area where providing a boolean flag (e.g. error: true) might be helpful (we may want both).

[kr8n3r]

formGroup: { classes: '' } is more flexible, though more verbose. Not sure what else whey'd want to add to the form group, but i'm probably mistaken.

would error :true additional option be more confusing? what if you want both classes and the error state?

[kr8n3r]

rewritten to accept formGroup: { classes: '' } way of passing options to the macro

[36degrees]

LGTM 👍