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_votes_controller.rb

@@ -3,7 +3,7 @@ class PostVotesController < ApplicationController
 
   def index
     @post_votes = authorize PostVote.visible(CurrentUser.user).paginated_search(params, count_pages: true)
-    @post_votes = @post_votes.includes(:user, post: :uploader) if request.format.html?
+    @post_votes = @post_votes.includes(:user, post: [:uploader, :media_asset]) if request.format.html?
 
     respond_with(@post_votes)
   end