Allow deleting/halting of messages

[jclem]

This allows users to delete any message, and to stop model output.

• To delete: Right click a message and click "Delete message"
• To stop output: Right click an in-progress message and click "Stop output"

When output is stopped, the message that has been streamed so far is persisted, and then can be optionally deleted.

https://user-images.githubusercontent.com/412282/234904862-315d1e64-4aff-4103-9e86-f3d7b92eb94d.mp4