Context: https://linear.app/comm/issue/ENG-761/spacing-regression-between-conversations-in-messagelist
Previous approach: https://phab.comm.dev/D3537
There was a regression with the `MessageList` component where the spacing between the `conversationHeader` (AKA timestamp) and messages was removed.
Went through the git history of `chat-message-list.css` and checked to see if any commits changed the `div.conversationHeader` CSS selector and found D3067 to be relevant. Specifically, the diff removed the `padding` property (I believe unintentionally).
I reintroduced the `padding` property, but set vertical padding to `6px` instead of `7px` to maintain the relationship with `font-size` (which went from `14px` to `12px`).