Added FeedOnlyMode to support NuGetV2RepositoryMirrors

Added FeedOnlyMode to support NuGetV2RepositoryMirrors

1) TODO: Always prevent user registration in the FeedOnlyMode
Done. Also, refactored code a little bit to move "feedOnlyMode" check to RegisterRoutes method in Routes.cs which calls RegisterUIRoutes, RegisterAPIV2Routes and so on. This helps ensure that existing unit tests call RegisterAPIV2Routes by calling RegisterRoutes(). RegisterServiceRoutes is still called from AppActivator.cs, so that, existing unit tests do not fail