**PLEASE NOTE THAT THIS DIFF AND SUBSEQUENT DIFFS IN THIS STACK WILL NOT BE LANDED UNTIL MORE OF THE REDESIGN IS READY SINCE THIS WILL CAUSE REGRESSIONS IN PROD**
This diff updates the chat message list ui to match the ui in the new web app redesign. In this diff I did the following:
- I removed the set background color so that the thread message list will inherit the panel background color
- Added 8px of horizontal padding to the messages
- Added 16px to the bottom of the message list container
Depends on D10564