Move summary list border from cells to rows

Simplify the code by moving the bottom border to the row itself. This requires us to set `border-collapse: collapse` in order to use the collapsing border model [1] as in the separated borders model (the initial border model) – 'rows, columns, row groups, and column groups cannot have borders' [2].

Unfortunately some browsers, including IE11 and Safari, still omit the border where a row is missing cells – so we do still need to provide the empty pseudo-element 'cell' for rows without actions.

[1]: https://drafts.csswg.org/css2/#collapsing-borders
[2]: https://drafts.csswg.org/css2/#:~:text=Rows%2C%20columns%2C%20row%20groups%2C%20and%20column%20groups%20cannot%20have%20borders%20(i.e.%2C%20user%20agents%20must%20ignore%20the%20border%20properties%20for%20those%20elements).

src/govuk/components/summary-list/_index.scss

@@ -6,27 +6,28 @@
       display: table;
       width: 100%;
       table-layout: fixed; // Required to allow us to wrap words that overflow.
+      border-collapse: collapse;
     }
     margin: 0; // Reset default user agent styles
     @include govuk-responsive-margin(6, "bottom");
   }
 
   .govuk-summary-list__row {
+    border-bottom: 1px solid $govuk-border-colour;
+
     @include govuk-media-query($until: tablet) {
       margin-bottom: govuk-spacing(3);
-      border-bottom: 1px solid $govuk-border-colour;
     }
     @include govuk-media-query($from: tablet) {
       display: table-row;
     }
   }
 
   // Provide an empty 'cell' for rows that don't have actions – otherwise the
-  // bottom border is not drawn for that part of the row.
+  // bottom border is not drawn for that part of the row in some browsers.
   .govuk-summary-list__row--no-actions:after {
     content: "";
     display: table-cell;
-    border-bottom: 1px solid $govuk-border-colour;
   }
 
   .govuk-summary-list__key,
@@ -39,7 +40,6 @@
       padding-top: govuk-spacing(2);
       padding-right: govuk-spacing(4);
       padding-bottom: govuk-spacing(2);
-      border-bottom: 1px solid $govuk-border-colour;
     }
   }
 
@@ -117,36 +117,30 @@
 
   // No border on entire summary list
   .govuk-summary-list--no-border {
-    @include govuk-media-query($until: tablet) {
-      .govuk-summary-list__row {
-        border: 0;
-      }
+    .govuk-summary-list__row {
+      border: 0;
     }
 
+    // Increase padding by 1px to compensate for 'missing' border
     @include govuk-media-query($from: tablet) {
       .govuk-summary-list__key,
       .govuk-summary-list__value,
       .govuk-summary-list__actions {
-        // Remove 1px border, add 1px height back on
         padding-bottom: govuk-spacing(2) + 1px;
-        border: 0;
       }
     }
   }
 
   // No border on specific rows
   .govuk-summary-list__row--no-border {
-    @include govuk-media-query($until: tablet) {
-      border: 0;
-    }
+    border: 0;
 
+    // Increase padding by 1px to compensate for 'missing' border
     @include govuk-media-query($from: tablet) {
       .govuk-summary-list__key,
       .govuk-summary-list__value,
       .govuk-summary-list__actions {
-        // Remove 1px border, add 1px height back on
         padding-bottom: govuk-spacing(2) + 1px;
-        border: 0;
       }
     }
   }