Add support for `max_posts` query param. Also make it easier to let

Ember support query params for sorting our tables. Just add it in one
place.

app/assets/javascripts/discourse/controllers/discovery-sortable.js.es6

@@ -1,10 +1,24 @@
 import DiscourseController from 'discourse/controllers/controller';
 
-export default DiscourseController.extend({
+// Just add query params here to have them automatically passed to topic list filters.
+export var queryParams = {
+  order: { replace: true },
+  ascending: { replace: true },
+  status: { replace: true },
+  state: { replace: true },
+  search: { replace: true },
+  max_posts: { replace: true }
+};
+
+// Basic controller options
+var controllerOpts = {
   needs: ['discovery/topics'],
-  queryParams: ['order', 'ascending', 'status', 'state', 'search'],
-  order: Em.computed.alias('controllers.discovery/topics.order'),
-  ascending: Em.computed.alias('controllers.discovery/topics.ascending'),
-  status: Em.computed.alias('controllers.discovery/topics.status'),
-  search: Em.computed.alias('controllers.discovery/topics.search')
+  queryParams: Ember.keys(queryParams)
+};
+
+// Aliases for the values
+controllerOpts.queryParams.forEach(function(p) {
+  controllerOpts[p] = Em.computed.alias('controllers.discovery/topics.' + p);
 });
+
+export default DiscourseController.extend(controllerOpts);

app/assets/javascripts/discourse/controllers/discovery/topics.js.es6

@@ -1,14 +1,13 @@
 import DiscoveryController from 'discourse/controllers/discovery';
+import { queryParams } from 'discourse/controllers/discovery-sortable';
 
-export default DiscoveryController.extend({
+var controllerOpts = {
   needs: ['discovery'],
   bulkSelectEnabled: false,
   selected: [],
 
   order: 'default',
   ascending: false,
-  status: null,
-  search: null,
 
   actions: {
 
@@ -149,4 +148,13 @@ export default DiscoveryController.extend({
   loadMoreTopics: function() {
     return this.get('model').loadMore();
   }
+};
+
+Ember.keys(queryParams).forEach(function(p) {
+  // If we don't have a default value, initialize it to null
+  if (typeof controllerOpts[p] === 'undefined') {
+    controllerOpts[p] = null;
+  }
 });
+
+export default DiscoveryController.extend(controllerOpts);

app/assets/javascripts/discourse/routes/build-topic-route.js.es6

@@ -1,12 +1,6 @@
 // A helper to build a topic route for a filter
 
-export var queryParams = {
-  sort: { replace: true },
-  ascending: { replace: true },
-  status: { replace: true },
-  state: { replace: true },
-  search: { replace: true }
-};
+import { queryParams } from 'discourse/controllers/discovery-sortable';
 
 export function filterQueryParams(params, defaultParams) {
   var findOpts = defaultParams || {};