Closes #12, Closes #13, Closes #14, Closes #16, Closes #17, Closes #18; #40, #41;
CRUD event done
CRUD user done
Fixed mappings (multiple one-to-many relations)
An event has no longer a referenced owned. To see if a user owns an event you can check if an involved object exists where the user and the owner matches and where the Role is Role.OWNER
UI still needs improvements but that can be commited as an improvement. UI works so the issue is closed.

TESTED MANUALLY: create a user, edit user, delete user with events and with no events, create event, delete event, edit event. All working.
Feel free to test it.