[v3.x.x] Show "New Message" indicator when new messages are out of view in the chat UI

[JasonWeill]

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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:

• #64 (pin scrolling to bottom)
• #111 (notifications, such as sound effects or badging, for new messages in chat)
• #176 (feedback that Jupyternaut is working)