compose: Distinguish get_message_type() from composing().

We now only call compose_state.composing() in a boolean context,
where we simply care whether or not the compose box is open.  The
function now also returns true/false.

Callers who need to know the actual message type (e.g. "stream" or
"private") now call compose_state.get_message_type().

static/js/drafts.js

@@ -72,7 +72,7 @@ exports.snapshot_message = function () {
 
     // Save what we can.
     var message = {
-        type: compose_state.composing(),
+        type: compose_state.get_message_type(),
         content: compose_state.message_content(),
     };
     if (message.type === "private") {