Improve project deletion

Moved from the discussion on https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14756#note_43044485

Problem

Currently, it needs to take some time to delete the project completely after the user clicks "Delete" button. So it would have 2 problems:

• How the user know the project has been deleted successfully.
• After click "delete", if the user immediately creates a new project with the same name, how the user know it cannot be created is because the old project hasn't been deleted successfully.