Add margin below final element in the Error Summary

[romaricpascal]

What

Add back a margin after the final element of the content in the Error Summary (either the final error in the list or the description).

Note: it might be that the margin does not necessarily need to be added back on the <li> it was removed from. The content is enclosed in the govuk-error-summary__body.

Why

We recently removed a 5px margin under the final <li> element in #4971, but it turns out that the extra margin can be helpful to account for descenders of letters like 'g', 'y', 'p'...

Who needs to work on this

Developers, designers

Who needs to review this

Developers, designers

Done when

• Margins have been added back at the end of Error summary's content