Event signup bug

[navro]

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.