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/views/pool_orders/edit.html.erb

@@ -12,7 +12,7 @@
         <%= render "posts/partials/common/inline_blacklist" %>
 
         <ul id="sortable">
-          <% @pool.posts.each do |post| %>
+          <% @pool.posts.includes(:media_asset).each do |post| %>
             <%= tag.li id: "pool[post_ids]_#{post.id}" do -%>
               <%= post_preview(post, show_deleted: true).presence || "Hidden: Post ##{post.id}" -%>
             <% end -%>