Added option to disable deleting of non empty repositories through the u...

Hello, i dont really like the idea of using federation to protect repositories against unintentional deletion as suggested in: https://groups.google.com/forum/#!topic/gitblit/qfsxQSUD79c.
This commit adds a new option which disables the delete link if the repository is not empty. If the administrator wishes to delete such a repository, he needs to delete it from the filesystem.

Best regards,
Peter Mihalik