Fix box overlap causing links to be non-clickable
Fix box overlap causing links to be non-clickable

 - The `line-height` of the large letters was causing an invisble overlap with
   any links directly below it. This meant that the links were not clickable,
   as the large box of the large letter was being clicked.
 - Removing the bottom margin reset fixes this, as the default bottom margin
   pushes the content below the letter down enough to stop any overlap.

alphagov/frontend,alphagov/frontend,alphagov/frontend,alphagov/frontend