backups: add stub backup service.

app/logical/backup_service.rb

@@ -0,0 +1,5 @@
+class BackupService
+  def backup(file_path, options = {})
+    raise NotImplementedError.new("#{self.class}.backup not implemented")
+  end
+end

app/logical/null_backup_service.rb

@@ -0,0 +1,5 @@
+class NullBackupService
+  def backup(file_path, options = {})
+    # do nothing
+  end
+end