Fix spacing between govspeak headings and attachment

- headings don't have margin bottom in govspeak, so rely on elements following them to have top margin for spacing
- when followed by an attachment, this is a problem because an attachment is a gem component and therefore doesn't have top margin
- solution is to add bottom margin to headings that does not exceed the top margin of other elements, so if both are present they will overlap without a visual difference
- but for attachments the space will be provided by this new bottom margin
- only doing this for H2 for now as this is the only example of this problem in the wild and I'm hesitant to make sweeping changes to govspeak

app/assets/stylesheets/govuk_publishing_components/components/helpers/_markdown-typography.scss

@@ -31,12 +31,11 @@
 
   h2 {
     margin-top: $govuk-gutter;
-    margin-bottom: 0;
+    margin-bottom: govuk-spacing(4);
     @include govuk-font($size: 27, $weight: bold);
 
     @include govuk-media-query($from: desktop) {
       margin-top: $govuk-gutter * 1.5;
-      margin-bottom: 0;
     }
   }