posts: use storage manager to backup files.

* Perform backups synchronously inside `distribute_files` instead of
  asynchronously in `queue_backup`. Asynchronous backups assumed that
  files are stored on the local filesystem, which isn't true in general.

* Remove obsolete backup service classes.

test/test_helper.rb

@@ -41,6 +41,7 @@ class ActiveSupport::TestCase
 
     storage_manager = StorageManager::Local.new(base_dir: "#{Rails.root}/public/data/test")
     Danbooru.config.stubs(:storage_manager).returns(storage_manager)
+    Danbooru.config.stubs(:backup_storage_manager).returns(StorageManager::Null.new)
   end
 
   teardown do