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/uploads_controller.rb

@@ -26,7 +26,7 @@ class UploadsController < ApplicationController
 
   def index
     @uploads = authorize Upload.visible(CurrentUser.user).paginated_search(params, count_pages: true)
-    @uploads = @uploads.includes(:uploader, post: :uploader) if request.format.html?
+    @uploads = @uploads.includes(:uploader, post: [:media_asset, :uploader]) if request.format.html?
 
     respond_with(@uploads)
   end