Fix layout_header CSS issues

Currently, the layout header component looks different in the context of
static vs in isolation.
- on the account manager, which does not use static, the layout header
navigation does not align as intended with the rest of the header on
tablet devices
- in the context of an application that uses static, the layout header
  inherits some styles from static. Additionally, application CSS will
conflict with the layout_header CSS resulting in spacing issues.

This aims to address some of these problems by removing unnecessary
classes, as well as increasing layout_header CSS specificity where this
CSS might be overruled by external styles from either static or the
parent application's stylesheet.

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

@@ -31,7 +31,7 @@
 }
 
 .gem-c-layout-header--search-left {
-  .gem-c-header__menu-button {
+  .gem-c-header__menu-button.govuk-header__menu-button {
     margin-top: - govuk-spacing(7);
     left: 0;
   }
@@ -70,18 +70,24 @@
       margin-top: 0;
     }
 
-    .gem-c-header__content {
+    .gem-c-header__content.govuk-header__content {
       @include govuk-grid-column(two-thirds);
       padding-left: govuk-spacing(6);
       padding-right: govuk-spacing(1);
     }
   }
 }
 
+.gem-c-layout-header__search.govuk-grid-column-one-third-from-desktop,
+.gem-c-layout-header__logo.govuk-grid-column-one-third-from-desktop,
+.gem-c-layout-header__search.govuk-grid-column-one-third,
+.gem-c-layout-header__logo.govuk-grid-column-two-thirds {
+  padding-right: 0;
+  padding-left: 0;
+}
+
 .gem-c-layout-header__logo,
 .gem-c-layout-header__search {
-  padding: 0;
-
   @include govuk-media-query($until: "tablet") {
     margin-bottom: govuk-spacing(3);
   }
@@ -96,8 +102,9 @@
   }
 }
 
-.gem-c-header__content {
+.gem-c-header__content.govuk-header__content {
   width: auto;
+  padding-left: 0;
 
   @include govuk-media-query($from: desktop) {
     float: right;
@@ -159,3 +166,12 @@
     display: block;
   }
 }
+
+.govuk-header__menu-button.gem-c-header__menu-button {
+  top: govuk-spacing(4);
+  right: 0;
+}
+
+.gem-c-header__nav {
+  clear: both;
+}