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_versions_controller.rb

@@ -9,7 +9,7 @@ class PostVersionsController < ApplicationController
     @post_versions = authorize PostVersion.paginated_search(params)
 
     if request.format.html?
-      @post_versions = @post_versions.includes(:updater, post: [:uploader, :versions])
+      @post_versions = @post_versions.includes(:updater, post: [:uploader, :media_asset, :versions])
     else
       @post_versions = @post_versions.includes(post: :versions)
     end