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/controllers/posts_controller.rb

@@ -26,9 +26,11 @@ class PostsController < ApplicationController
       include_deleted = @post.is_deleted? || (@post.parent_id.present? && @post.parent.is_deleted?) || CurrentUser.user.show_deleted_children?
       @sibling_posts = @post.parent.present? ? @post.parent.children : Post.none
       @sibling_posts = @sibling_posts.undeleted unless include_deleted
+      @sibling_posts = @sibling_posts.includes(:media_asset)
 
       @child_posts = @post.children
       @child_posts = @child_posts.undeleted unless include_deleted
+      @sibling_posts = @sibling_posts.includes(:media_asset)
     end
 
     respond_with(@post) do |format|