compose: Fix behavior hitting enter with the preview area open.

The new behavior is:
(1) If enter-sends is enabled, just send the messsage.
(2) If enter-sends is not enabled, return focus to the compose area.

Based on great work by khantaalaman in #3673.

Fixes #3489.

static/js/compose.js

@@ -607,6 +607,17 @@ function send_message(request) {
     }
 }
 
+exports.enter_with_preview_open = function () {
+    clear_preview_area();
+    if (page_params.enter_sends) {
+        // If enter_sends is enabled, we just send the message
+        send_message();
+    } else {
+        // Otherwise, we return to the compose box and focus it
+        $("#new_message_content").focus();
+    }
+};
+
 exports.respond_to_message = function (opts) {
     var message;
     var msg_type;