feat(engine/bpmn): add message end event
- message end event has the same behaviour as for service task
- add test cases

related to CAM-436
fix(cockpit/procdef): apply variable filter using EXISTS
Merge branch 'master' of github.com:camunda/camunda-bpm-platform
Merge branch 'CAM-436-MessageEndEvent' of https://github.com/polenz/camunda-bpm-platform into polenz-CAM-436-MessageEndEvent
Merge branch 'master' of github.com:camunda/camunda-bpm-platform
chore(webapps-it): add screenshot rule for selenium testing
chore(webapps-it): check if POST createInitialUser is successful