Add a pagination override to fix Bootstrap z-index issue

Bootstrap adds a `z-index: 2` to active pagination items.
Why they did this is not very clear. I would consider this a bug,
because no other pagination element has any `z-index` set and I do not
see any visual difference here. Leaving the `z-index` wis problematic
if we implement a sticky header for instance.

backend/app/assets/stylesheets/spree/backend/components/_pagination.scss

@@ -0,0 +1,5 @@
+.pagination .page-item.active .page-link {
+  // overwrite a weird `z-index: 2` from Bootstrap,
+  // so it does not peak out of the sticky header
+  z-index: auto;
+}

backend/app/assets/stylesheets/spree/backend/spree_admin.scss

@@ -29,6 +29,7 @@
 @import 'spree/backend/components/list_group';
 @import 'spree/backend/components/messages';
 @import 'spree/backend/components/progress';
+@import 'spree/backend/components/pagination';
 @import 'spree/backend/components/table-filter';
 @import 'spree/backend/components/navigation';
 @import 'spree/backend/components/sidebar';