posts: refactor post previews to use ViewComponent.

Refactor the post preview html to use the ViewComponent framework. This
lets us encapsulate all the HTML, CSS, and helper methods for a UI
component in a single place.

See https://viewcomponent.org.

config/webpacker.yml

@@ -11,7 +11,7 @@ default: &default
 
   # Additional paths webpack should lookup modules
   # ['app/assets', 'engine/foo/app/assets']
-  resolved_paths: []
+  additional_paths: ["app/components"]
 
   # Reload manifest.json on all requests so we reload latest compiled packs
   cache_manifest: false