Refine auto-configuration for Spring Social to:

- Only auto-configure FB/Twitter/LinkedIn if the `app-id` property is
  set.
- Only configure ConnectController and ProviderSignInController if
  there is a ConnectionFactoryLocator.
- Auto-configure Spring Social's SpringSocialDialect for Thymeleaf if
  Thymeleaf is present.
- Added several tests around Spring Social auto-configuration.

Fixes gh-1118