Indicate whether the compose box is in editing mode

[preetmishra]

Currently, it is difficult to determine whether a user is composing a new message or editing an existing message.

To enhance the user experience, we could indicate whether the compose box is in editing mode.

[neiljp]

@preetmishra For streams we now have the topic-change box, but did you have specific ideas on how to indicate this?

[preetmishra]

@neiljp We could potentially have a specific variety for this in #782.