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/logical/post_query_builder.rb

@@ -489,10 +489,10 @@ class PostQueryBuilder
     relation
   end
 
-  def build
+  def build(includes: nil)
     validate!
 
-    relation = Post.all
+    relation = Post.includes(includes)
     relation = add_joins(relation)
     relation = metatags_match(metatags, relation)
     relation = tags_match(tags, relation)
@@ -514,8 +514,8 @@ class PostQueryBuilder
     relation
   end
 
-  def paginated_posts(page, small_search_threshold: Danbooru.config.small_search_threshold.to_i, **options)
-    posts = build.paginate(page, **options)
+  def paginated_posts(page, small_search_threshold: Danbooru.config.small_search_threshold.to_i, includes: nil, **options)
+    posts = build(includes: includes).paginate(page, **options)
     posts = optimize_search(posts, small_search_threshold)
     posts.load
   end