Sort out re-use of event registration objects

In #484 (closed), it became apparent that Registration objects were expected to be recreated, but are currently being re-used (#484 (comment 34876)). We should look into this, and make it consistent.

Creating new registration objects when people re-register creates a more accurate timeline, but it may make things a bit harder to understand when someone occurs as both cancelled and registered (in that case, should we omit the cancelled registration from the view?).