[web] Fix spacing regression in MessageList component

Summary:
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).

Test Plan:
Things look as expected:
Before:

After:

Messed around with the web app and looked around the code and couldn't find any side effects.

Reviewers: abosh, varun, tomek, jacek, ashoat

Reviewed By: abosh, ashoat

Subscribers: adrian

Differential Revision: https://phab.comm.dev/D4777