posts: show length of videos and animations in thumbnails.

Show the length of videos and animated posts in the thumbnail. The
length is shown the top left corner in MM:SS format. This replaces the
play button icon.

Show a speaker icon instead of a music note icon for posts with sound.

Doing this requires doing `.includes(:media_asset)` in a bunch of
places to avoid N+1 queries when we access the post's duration.

app/components/post_preview_component/post_preview_component.html.erb

@@ -1,5 +1,18 @@
 <%= tag.article id: "post_#{post.id}", **article_attrs do -%>
   <%= link_to polymorphic_path(link_target, q: tags) do -%>
+    <% if is_animated? || has_sound? %>
+      <div class="post-animation-icon absolute top-0.5 left-0.5 p-0.5 leading-none text-xs">
+        <% if is_animated? %>
+          <span class="post-duration align-middle">
+            <%= duration_to_hhmmss(duration) %>
+          </span>
+        <% end %>
+        <% if has_sound? %>
+          <%= sound_icon(class: "h-3 mx-0.5") -%>
+        <% end %>
+      </div>
+    <% end %>
+
     <%= tag.picture do -%>
       <%= tag.source media: "(max-width: 660px)", srcset: cropped_url -%>
       <%= tag.source media: "(min-width: 660px)", srcset: post.preview_file_url -%>