Implemented posting rate-limit

Starting from like 117, we can just set lastPost to 0 if a user hasn't posted yet, and then compare the delta with Date.now(), like this:

         user.lastPost = user.lastPost || 0;
         if(Date.now() - user.lastPost < 86400000) {
           res.send(429, {error: 'Submissions are limited to once per 24 hour period'});
         } else {
           user.lastPost = new Date();
           // continue....

Then carry on from line 137 as you had it.