Strip underline from focused + hovered nav links

Our CSS for links in the navigation, which makes
the underline appear on hover is overriding the
GOVUK focus styles, making it look like there are
2 underlines. This adds some extra CSS to reset
it.

The new CSS is further down the stylesheet to the
hover styles. This gives them a higher specificity
meaning they override them when these links are
hovered over while in focus.

app/assets/stylesheets/components/navigation.scss

@@ -68,6 +68,10 @@
       text-decoration: underline;
     }
 
+    &:focus {
+      text-decoration: none; // override the :hover style (the focus style has its own underline)
+    }
+
   }
 
   &-service {
@@ -167,6 +171,10 @@
       text-decoration: underline;
     }
 
+    &:focus { // override the :hover style (the focus style has its own underline)
+      text-decoration: none;
+    }
+
     &.selected {
       @include bold-19;
       position: relative;