Redesign button indicator to be red

This has prompted some questions around exposing the button styling as this design essentially extends the button design and puts something inside it.

src/govuk/components/hide-this-page/_index.scss

@@ -7,6 +7,9 @@
     top: 0;
     left: 0;
     width: 100%;
+    // Copied approximately from the button sass. We need should expose the button
+    // box shadow and probably a bunch of other things defined locally to the button
+    box-shadow: 0 $govuk-border-width-form-element 0 govuk-shade($govuk-button-background-colour, 60%);
 
     @include govuk-media-query($from: tablet) {
       display: inline-block;
@@ -21,22 +24,22 @@
 
   .govuk-hide-this-page__button {
     margin-bottom: 0;
+    box-shadow: none;
   }
 
   .govuk-hide-this-page__indicator {
     @include govuk-responsive-padding(2);
-    display: block;
-    border: 1px solid $govuk-border-colour;
-    border-top: none;
+    display: none;
     opacity: 0;
-    color: $govuk-text-colour;
-    background-color: $govuk-body-background-colour;
+    color: govuk-colour("white");
+    background-color: govuk-colour("red");
     line-height: 0; // removes extra negative space below the indicators
     text-align: center;
     pointer-events: none;
   }
 
   .govuk-hide-this-page__indicator--visible {
+    display: block;
     opacity: 1;
   }