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_flags_controller.rb

@@ -10,7 +10,7 @@ class PostFlagsController < ApplicationController
     @post_flags = authorize PostFlag.paginated_search(params)
 
     if request.format.html?
-      @post_flags = @post_flags.includes(:creator, post: [:flags, :uploader, :approver])
+      @post_flags = @post_flags.includes(:creator, post: [:flags, :uploader, :approver, :media_asset])
     else
       @post_flags = @post_flags.includes(:post)
     end