Support toggling warning on/off.

Our application has an "idle timeout" warning which will log a user out + navigate away from the current page in the event they have been idle for too long. This requires us to have a programmatic way to disable the unsaved changes warning. I added an enabled flag and a method to the service to toggle the flag.

The test suite would not run...it looks like some of the dependencies with 'latest' version specified may have changed and caused some failures. I'd happily add tests once the test suite works.