Delete design repository when a project is destroyed.

This will need to be added to the Projects::DestroyService#trash_repositories!.

The destroy happens in 2 steps:

1. a move of the repository to a remporary place "#{path}+#{project.id}#{DELETED_FLAG}"
2. Async, the temporary directory is removed.

Also see Project::DestroyRollbackService which should handle restoring design repos if the destroy fails.

/cc @phikai