[6.x] Add a custom scrollbar for set pickers

[jaygeorge]

Description of the Problem

As per #13636 there's no hint that there are more sets out of view that you can choose from, so you probable won't try to scroll

What this PR Does

• Closes [6.x] Scroll hint for replicator/bard set picker #13636 by adding a CSS custom scrollbar that only appears when the container is scrollable
• Adjusts some bottom border-radius values very slightly to make the scrollbars blend in more

Before

After

How to Reproduce

1. Add a load of sets to Bard so that it's scrollable when opening the set picker