bug fix: Fix recent regression with at-mentioning.

One of my commits from yesterday erroneously set the
"mentioned" flag on messages that weren't mentioning
the current user, so you would get the pink/salmon
background when you sent at-mentions to other people.

Now we check the user_id before setting the flag.

frontend_tests/node_tests/echo.js

@@ -64,6 +64,14 @@ people.add({
     email: 'cordelia@zulip.com',
 });
 
+people.add({
+    full_name: 'Leo',
+    user_id: 102,
+    email: 'leo@zulip.com',
+});
+
+people.initialize_current_user(101);
+
 var stream_data = global.stream_data;
 var denmark = {
     subscribed: false,