fix sidebar messages going off-screen

[hchiam]

The user has to think of scrolling to see the latest messages. It should just show up without extra effort on the user's part, so they can focus on other actions.

To fix this, we could:

1. add messages to the top of the list instead of to the bottom, or
2. auto-move all messages up (same as auto-scroll to bottom) (sounds like we're going with this one), or
3. only show the last few messages.

These are technically possible with pure CSS, but it's much cleaner if we edit the JSX/HTML instead.

[hchiam]

Auto-scroll seems very troublesome to get working with <Drawer>. I'm going for adding to the top for now.