Update Grunt tasks
WICKET-5380 Wicket rebinds the SessionEntry session attribute and this causes problems in Glassfish

Bind SessionEntry as an attribute in the http session only when freshly created.
Do not set it on every request because this causes notifications in Glassfish which are too early for the logic this HttpSessionBindingListener is used for.
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.