By using `display: flex` and `flex-direction: column-reverse`, the chat
now automatically scrolls down when loaded, without requiring any
extra JavaScript.

We still need to scroll down with JavaScript when sending a message.

By using `column-reverse`, the messages container now works in reverse.
So the newest message is the first element in the container and the
oldest message is the last. This is the reverse of before.

Due to this, this change will likely break some plugins.