Fix snippet threads layout

What does this MR do and why?

Fixes snippet threads (sort of)

• Snippets seems to be very behind the other comments/notes across GitLab. It needs a lot of fixing 😅

What does this MR not do?

• Update Reply button (it's still using the old button as opposed to input)
• Fix Collapse thread styling (it looks like collapsible diff comments right now 🤷)
• Fix the connecting activity border when a regular comment comes after a thread
• Add a Reply button to comments

Screenshots or screen recordings

Before	After

How to set up and validate locally

UX Paper Cuts 16.2 → Settings UI (#398215 - closed)