Keeping the build files current and adding deep links to the Intializr.

Updated the Spring Boot version and the Gradle and Maven wrappers.

Also removed the Initializr image and updated the README to match.

I rewrote the document and edited the code (mostly changing package names) to use the Spring Initializr

I also upgraded the build files to Spring Boot 2.2.0, which necessitated changing the tests to use JUnit 5 and replacing the gradle wrappers.