Add post detail display markup

We make the component test pass by adding markup to display the post.

Inner green; outer red:
✘ Element .js-post-detail-title has text "Test Post"
✘ Element .js-post-detail-body has text "This post is a test!"

The acceptance test still has the same error! Why's that? Well, we're not actually saving the post when we click save, and we aren't loading the post on the show page.

app/templates/components/post-detail.hbs

@@ -1 +1,5 @@
-{{yield}}
+<h2 class="js-post-detail-title">{{post.title}}</h2>
+
+<div class="js-post-detail-body">
+  {{post.body}}
+</div>