put the Subscribe checkbox above Reply instead of above Bump

Pretty self-explanatory.  The choice of whether to subscribe to a thread happens when you first reply to it, so the Subscribe checkbox should be above the Reply button where we can easily see/click it, not above the Bump button all the way on the other side where it's easy to forget about it and end up subscribed to something you didn't want.

I don't really have a preference about whether that's done by moving the Subscribe checkbox to the right or the Reply button to the left.