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/helpers/components_helper.rb

@@ -4,6 +4,7 @@ module ComponentsHelper
   end
 
   def post_previews_html(posts, **options)
+    posts = posts.includes(:media_asset) if posts.is_a?(ActiveRecord::Relation)
     render PostPreviewComponent.with_collection(posts, **options)
   end