Small design adjustments

Increase bottom padding on buttons to 25px. Note that we only need 15px padding here, as another 10px comes from the invisible bottom border that improves high contrast mode presentation.

Add more spacing between the elements inside the button on mobile.

Fix alignment of IE8 show/hide icon.

Fixes #2324

src/govuk/components/accordion/_index.scss

@@ -171,10 +171,7 @@
         // IE8 fallback of icon with HTML symbol
         @include govuk-if-ie8 {
           content: "\25B2"; // "▲"
-
-          top: 0;
-          left: 0;
-
+          position: relative;
           border: 0;
         }
       }
@@ -197,7 +194,7 @@
     .govuk-accordion__section-button {
       width: 100%;
 
-      padding: govuk-spacing(2) 0 govuk-spacing(1) 0;
+      padding: govuk-spacing(2) 0 govuk-spacing(3) 0;
 
       border: 0;
 
@@ -320,7 +317,11 @@
     // As a result this is applied to the pseudo element that has a display:block property to maintain Design
     .govuk-accordion__section-summary:before,
     .govuk-accordion__section-toggle:before {
-      margin-top: 8px;
+      margin-top: govuk-spacing(2);
+
+      @include govuk-media-query ($from: desktop) {
+        margin-top: 8px;
+      }
     }
 
     // Space between icon and text,