Introduce security solution as part of WebGoat assignments

The XXE lesson contains, thanks to @zubcevic, the option to run in secure mode. Great idea!!! I think we can reuse this great idea more general: for each lesson present a insecure and secure solution this way we can:

• Base our documentation on this (use macros to fetch the secure code directly from the source code) which ensures the documentation in up-to-date.
• Running scanners against WebGoat should not find any vulnerability in secure mode :-)
• Just an idea: it will make it more interactive as people can code the secure solution themselves in a lab env. So we should be able to remove the secure solutions from WebGoat with a pre process step or something and then the assignment is make the unit test fail :-)

We should introduce this for more lessons.