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.

app/helpers/posts_helper.rb

@@ -1,8 +1,10 @@
 module PostsHelper
+  def post_preview(post, **options)
+    render PostPreviewComponent.new(post: post, **options)
+  end
+
   def post_previews_html(posts, **options)
-    posts.map do |post|
-      PostPresenter.preview(post, **options)
-    end.join("").html_safe
+    render PostPreviewComponent.with_collection(posts, **options)
   end
 
   def reportbooru_enabled?