Adjust spacing for back link

[frankieroberto]

The current default spacing for the back link component means that, when used in the correct position within the page template it appears too close to the header.

This adds some spacing before the back link, using the spacing given in the transaction page example in page templates, which currently manually adjusts the spacing by adding this class to the back link: nhsuk-u-margin-top-4.

Setting the default spacing before the link would mean that the override class doesn’t have to be added to every back link on every page. However the spacing could still be overridden if needed.

Screenshots

Before	After

Checklist

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