Edit Message box going out of focus on topic edit.

When editing a message, it is possible to lose the content of the one’s drafted edit mid-message as follows:

• Start editing a message
• Another user moves the topic, triggering a rerender of your view (preserving the message edit box, but likely losing your keyboard focus)
• You hit t or some other single-letter keyboard shortcut that navigates away from the current view.
• (The back button may show you your partially-composed edit, but this is very non obvious).

We should arrange to avoid losing keyboard focus. The logic for this is surely somewhere in message_edit.js.

[timabbott]

@LoopThrough-i-j I rewrote the description to provide a much more clear reproduction recipe.

[LoopThrough-i-j]

@timabbott, @amanagr please have a look when you have some time.