Convert Composer to use Store

- Uses store for `PostStream`, `TopicList` too

test/javascripts/helpers/create-store.js.es6

@@ -1,8 +1,10 @@
 import Store from "discourse/models/store";
 import RestAdapter from 'discourse/adapters/rest';
+import Resolver from 'discourse/ember/resolver';
 
 let _restAdapter;
 export default function() {
+  const resolver = Resolver.create();
   return Store.create({
     container: {
       lookup(type) {
@@ -12,7 +14,10 @@ export default function() {
         }
       },
 
-      lookupFactory: function() { }
+      lookupFactory(type) {
+        const split = type.split(':');
+        return resolver.customResolve({type: split[0], fullNameWithoutType: split[1]});
+      },
     }
   });
 }