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/post_replacements_controller.rb

@@ -24,7 +24,7 @@ class PostReplacementsController < ApplicationController
   def index
     params[:search][:post_id] = params.delete(:post_id) if params.key?(:post_id)
     @post_replacements = authorize PostReplacement.paginated_search(params)
-    @post_replacements = @post_replacements.includes(:creator, post: :uploader) if request.format.html?
+    @post_replacements = @post_replacements.includes(:creator, post: [:uploader, :media_asset]) if request.format.html?
 
     respond_with(@post_replacements)
   end