feat(scroll): show scrollbar on hover only option

[josephmcg]

What this PR does 📖
I initially made a huge PR that affected all scrolls on the app, but then we switched main branches. This is a small chunk of that large pr

• global scroll style rules
• add lighter color on hover for webkit (thanks Thomas)
• I think being able to hide the scrollbar until you hover/focus within an element is useful functionality, but I don't think it should be applied to every scrollbar in the entire app. I created a class (.hover-scroll) which will do this. for now, I added it to the chat sidebar. You can test by manually creating elements via the inspector to trigger the overflow

Which issue(s) this PR fixes 🔨
AP-1929

Special notes for reviewers 🗒️

Additional comments 🎤