compose: Allow message_id to be passed to respond_to_message.

Up until now, the currently selected message was the one that was always
responded to. Now if there's a message_id passed in, we'll use that
message instead, otherwise we'll fall back to the selected message.

This is a prep commit for allowing quoting and replying while editing
a message sent earlier.

web/src/compose_reply.js

@@ -42,7 +42,8 @@ export function respond_to_message(opts) {
         }
         message = message_opts.message;
     } else {
-        message = message_lists.current.selected_message();
+        message =
+            message_lists.current.get(opts.message_id) || message_lists.current.selected_message();
 
         if (message === undefined) {
             // empty narrow implementation