Added uniqueness validation to user, scoped to event.
Added nested attribute uniqueness for user names to event model.

Because when users are created via nested attributes, they are not yet
saved when validation is checked (so normal uniqueness validation does
not work).
Fixed some event tests.

After adding nested_attributes_uniqueness_validator, some tests needed
the event to be reloaded first.
Custom users update method for event controller.

We can't rely on accepts_nested_attributes_for to destroy and update
users without messing around with validations too much.