Support for a /last route to go to the last post in a topic.

app/assets/javascripts/discourse/models/topic.js

@@ -69,7 +69,11 @@ Discourse.Topic = Discourse.Model.extend({
   urlForPostNumber: function(postNumber) {
     var url = this.get('url');
     if (postNumber && (postNumber > 1)) {
-      url += "/" + postNumber;
+      if (postNumber >= this.get('highest_post_number')) {
+        url += "/last";
+      } else {
+        url += "/" + postNumber;
+      }
     }
     return url;
   },

app/assets/javascripts/discourse/routes/topic_from_params_route.js

@@ -33,6 +33,9 @@ Discourse.TopicFromParamsRoute = Discourse.Route.extend({
     var topicController = this.controllerFor('topic'),
         composerController = this.controllerFor('composer');
 
+    // I sincerely hope no topic gets this many posts
+    if (params.nearPost === "last") { params.nearPost = 999999999; }
+
     postStream.refresh(params).then(function () {
       // The post we requested might not exist. Let's find the closest post
       var closest = postStream.closestPostNumberFor(params.nearPost) || 1;