rake: add images:backup rake task.

lib/tasks/images.rake

@@ -1,2 +1,16 @@
 namespace :images do
+  desc "Backup images"
+  task :backup => :environment do
+    CurrentUser.user = User.system
+    sm = Danbooru.config.backup_storage_manager
+    tags = ENV["BACKUP_TAGS"]
+    posts = Post.tag_match(tags)
+
+    posts.find_each do |post|
+      sm.store_file(post.file(:preview), post, :preview) if post.has_preview?
+      sm.store_file(post.file(:crop), post, :crop) if post.has_cropped?
+      sm.store_file(post.file(:sample), post, :sample) if post.has_large?
+      sm.store_file(post.file(:original), post, :original)
+    end
+  end
 end