Fix checking if the target folder exists on uploading releases to forks
    
    ``werkzeug.secure_filename()`` would convert /forks/user/project into
    forks_user_project making the check for the target folder invalid.
    Thus we're removing it. The regex used when creating the project should
    cover our back on insuring the project name is safe/secure.
    
    Fixes https://pagure.io/pagure/issue/1262