Show "New Message" indicator when new messages are out of view in the chat UI

[JasonWeill]

Problem

When the user is scrolled somewhere other than the bottom of the chat UI's messages panel, and a new message comes in, there is no visual indicator that new messages have arrived.

Proposed Solution

Add a "New Message" indicator, which, when clicked, jumps to either the bottom of the messages panel or to the first new message received. The indicator may indicate how many new messages have arrived.

Additional context

See also:

• Scroll pinning in Chat UI #64 (pin scrolling to bottom)
• Notifications when chat UI sends a response message #111 (notifications, such as sound effects or badging, for new messages in chat)
• Provide feedback that Jupyternaut is working #176 (feedback that Jupyternaut is working)