Should not allow user to delete default permission scheme

When there is no project associated with the default permission scheme, the permission scheme admin page shows up a "delete" link to allow user to delete the default permission scheme.

However, once the default permission scheme is deleted, many functionalities will break, including: fail to instantiate a new project. Recreating a permission scheme named "default" won't fix the problem, as the code assumes the default permission scheme has id of value 1.