Upgrade Jetty and JUnit for the quickstart of 6.x
WICKET-5381 Add Wicket.Event.unsubscribe method

Additionally update Grunt configs (copy from master branch) to be able to run the tests
WICKET-5381 Add Wicket.Event.unsubscribe method

Extract the topic names as constants.
Optimize the lookups to Wicket.Event.xyz() methods.