We use a hack to extend the focus style of single
links in template list items (those not part of a
path of links).
This extended the 'focus box' downwards so it
covered the hint text below the link by the height
of the link (which is block-level).
Problems happen if the link wraps to multiple
lines. The hint is always on one line so the focus
looks over-extended.
These changes guard against those problems by
using the line-height instead of the block-height.
They include adding a Sass function to reference
the line-heights in GOVUK Frontend's Sass API:
https://frontend.design-system.service.gov.uk/sass-api-reference/#govuk-typography-scale