Add base styling for footer actions to support simple customisations

- Fixes #11629
- Relates to clean up done as part of #11629 which removed a lot of styling for footer actions which appeared to be unused.
- Re-add some base styling to support the ability for basic customisations where buttons are added within the `extra_footer_actions` block.

client/scss/components/_footer.scss

@@ -89,7 +89,18 @@
   }
 
   &--primary {
+    // Support basic layout support for items added via `extra_footer_actions`
+
+    display: grid;
+    gap: theme('spacing.2');
+
+    > * {
+      // Reset the margin on any .button sibling elements
+      margin-inline-start: initial;
+    }
+
     @include media-breakpoint-up(sm) {
+      grid-auto-flow: column;
       width: 310px;
     }
   }