Event signup bug

#### Description
If the group of an `event_user` is updated through the admin interface, the position of that `event_user` might be changed to the beginning of the queue. This is not very good...

#### Expected behaviour
Changing the group of an `event_user` through the admin interface has no effect on the `event_users` position in the queue.

#### Actual behaviour
In some cases, the `event_user` is moved to the beginning of the queue.

#### Steps to reproduce
1. Create a new event signup without priorities, and with at least 2 slots.
2. Let user `X` sign up for the event.
3. Let user `Y` sign up for the event.
4. Visit the admin page for this event signup and select user `Y` in the attendance list.
5. The user should have position 2.
6. Update the group name for user `Y`.
7. The user will now get position 1.

#### Possible explanation
When the `event_user` is updated, the `user_type` also changes from the default value of `nil` to `''`. This is probably because we allow admins to change user types for events that allow different priorities. For some reason the form still changes this field when no priorities are allowed. It's probably bad that we allow both `nil` and `''` to represent the user type `other`.