Hook routes into post model

This acceptance test error drives a lot of logic: to display the post's titl
e on the show page, we need to save the post on the new page, include the ID in
the transition to the show route, then load the post on the show page's model ho
ok.

Outer red: No model was found for 'post'

With this logic added, the acceptance test errors out quickly: there _is_ no
 `post` model.

app/controllers/posts/new.js

@@ -2,8 +2,11 @@ import Controller from '@ember/controller';
 
 export default Controller.extend({
   actions: {
-    savePost() {
-      this.transitionToRoute('posts.show');
+    savePost(postData) {
+      let post = this.store.createRecord('post', postData);
+      post.save().then((post) => {
+        this.transitionToRoute('posts.show', post.id);
+      });
     }
   }
 });

app/router.js

@@ -9,7 +9,7 @@ const Router = EmberRouter.extend({
 Router.map(function() {
   this.route('posts', function() {
     this.route('new');
-    this.route('show');
+    this.route('show', { path: ':id' });
   });
 });
 

app/routes/posts/show.js

@@ -1,4 +1,7 @@
 import Route from '@ember/routing/route';
 
 export default Route.extend({
+  model(params) {
+    return this.store.findRecord('post', params.id);
+  }
 });