Add a max setTimout for expiring messages (over max == immediate)

Discovered a user log where expiring message checks were happening
constantly. This ensures that a very large timeout doesn't roll over
into immediate callbacks.

FREEBIE

js/expiring_messages.js

@@ -31,8 +31,13 @@
             console.log('next message expires', new Date(expires_at).toISOString());
 
             var wait = expires_at - Date.now();
+
+            // In the past
             if (wait < 0) { wait = 0; }
 
+            // Too far in the future, since it's limited to a 32-bit value
+            if (wait > 2147483647) { wait = 2147483647; }
+
             clearTimeout(timeout);
             timeout = setTimeout(destroyExpiredMessages, wait);
         });