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/comments_controller.rb

@@ -99,7 +99,7 @@ class CommentsController < ApplicationController
       @comments = @comments.includes(:creator, :post)
       @comments = @comments.select { |comment| comment.post.visible? }
     elsif request.format.html?
-      @comments = @comments.includes(:creator, :updater, post: :uploader)
+      @comments = @comments.includes(:creator, :updater, post: [:uploader, :media_asset])
       @comments = @comments.includes(:votes) if CurrentUser.is_member?
     end