Improve logo spacing

The new search box is larger than the old one making the header section taller.
This adjusts the header to put the searchbox central vertically and aligns the
logo with it.

app/assets/stylesheets/govuk_publishing_components/components/_layout-header.scss

@@ -31,7 +31,6 @@
 .gem-c-layout-header__logo,
 .gem-c-layout-header__search {
   padding: 0;
-  margin-bottom: govuk-spacing(2);
 
   @include govuk-media-query($until: "tablet") {
     margin-bottom: 0;
@@ -40,6 +39,8 @@
 
 .gem-c-header__logo {
   white-space: nowrap;
+  margin-top: govuk-spacing(1);
+  margin-bottom: govuk-spacing(3);
 }
 
 .gem-c-header__content {