Fix JDBC samples to ensure DataSource is initialized

Ideally Spring Boot would initialize the data source for us (see
https://github.com/spring-projects/spring-boot/issues/1115), but we
can work around it with @DependsOn. The main new thing in this
approach though is to use a GlobalAuthenticationConfigurerAdapter
to configure the authentication manager (much better than using
a WebSecurityConfigurerAdapter)

Fixes gh-222