Added FeedOnlyMode to support NuGetV2RepositoryMirrors

Added FeedOnlyMode to support NuGetV2RepositoryMirrors

1) TODO: Always prevent user registration in the FeedOnlyMode
This is more of a defense-in-depth. If it were possible to hit the Register method if the ControllerName and ActionName are known, by any means, it is prevented. We only need to stop Register from executing, since, unregistered users can never push a package. With other methods though it is not a concern