Add fix script to regenerate old posts with transparent backgrounds.

Fixes #1862. Very old posts with transparent backgrounds used black
instead of white backgrounds in thumbnails. This was changed in #603
(see also #1239), but the thumbnails were never regenerated.

script/fixes/068_regenerate_transparent_backgrounds.rb

@@ -0,0 +1,7 @@
+#!/usr/bin/env ruby
+
+require_relative "../../config/environment"
+
+Post.system_tag_match("transparent_background id:<=1361925").find_each do |post|
+  post.regenerate_later!("resizes", User.system)
+end