[BUGFIX] Fixed invalid Exception class

The setSlug() method in the Event domain model is now throwing an
InvalidArgumentException (before it was a InvalidArgument, which does
not exist).
[BUGFIX] Fixed method call in group view helper

The groupByOrganzier() method does not exist and was replaced with the
existing groupByOrganizer() method.
[!!!][TASK] Unified address handling

The address properties for the location and the organizer of an event
have been removed from the event domain model and have been replaced
with an inline address record.

This makes enhancements of the address records easier in the future.

The inline address records are hidden in the record list by a hook.