Update label margins to match fieldsets and align with GOV.UK

[edwardhorsford]

Description

This updates the bottom margins of labels to better match those of legends. I've followed a bit of how GOV.UK structures these.

At the moment I think the label margins are overly large - too separated from the hint or inputs they relate to. GOV.UK doesn't have responsive margins for these - the label margins are all fixed and the same as the fieldset legends.

The only label that doesn't now match fieldset legends is the small size - where it follows the no modifier size 1.

XL label before

XL label after

L label before

L label after

M label before

M label after

S label before

S label after

No modifier label before

No modifier label after

Fixes #947

Checklist

• Tested against our testing policy (Resolution, Browser & Accessibility)
• Follows our coding standards and style guide
• CHANGELOG entry

[anandamaryon1]

Sensible change, helps to group labels with their inputs and now matches fieldset legends. Looks good thank you.

[anandamaryon1]

Good catch with the backstop reference images