Previously, SocialWebAutoConfiguration would create a
SpringSocialDialect bean when SpringTemplateEngine was on the
classpath. This class exists in both Thymeleaf 2 and Thymeleaf 3 but
SpringSocialDialect is only compatible with Thymeleaf 2.

This commit updates the auto-configuration to require
SpringResourceResourceResolver to be on the classpath. This class
exists in Thymeleaf 2 but does not exist in Thymeleaf 3.

Closes gh-4858