Add ProviderSignInController to Spring Social autoconfig

This adds ProviderSignInController to Spring Social autoconfig and fixes a problem with the configuration of the UserIdSource bean.

The Spring Social Showcase sample has been reworked to use Spring Boot and take advantage of the code in this PR. The change is in the boot-autoconfig branch at https://github.com/spring-projects/spring-social-samples/tree/boot-autoconfig/spring-social-showcase.