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/views/comments/_index_by_post.html.erb

@@ -5,7 +5,7 @@
 
   <% @posts.select(&:visible?).each do |post| %>
     <% if post.comments.unhidden(CurrentUser.user).any? || post.comments.hidden(CurrentUser.user).any? %>
-      <%= content_tag(:div, { id: "post_#{post.id}", class: ["post", *PostPresenter.preview_class(post)].join(" ") }.merge(PostPresenter.data_attributes(post))) do %>
+      <%= tag.div id: "post_#{post.id}", **PostPreviewComponent.new(post: post).article_attrs("post") do %>
         <div class="preview">
           <%= link_to(image_tag(post.preview_file_url), post_path(post)) %>
         </div>