Don't clear comment text box when submitting

It's a pretty minor thing, but it'd be nice if we could wait with clearing the text box when submitting a comment until the server has acknowledged the message. Especially on mobile it can be annoying if you lose a comment that you've just laboured over for the last 5 minutes.

The button can probably be disabled on submit (and should be re-enabled on failure/success) to prevent double submitting something