Sort out re-use of event registration objects

Ghost User
2018-11-14T11:12:03+01:00

In #484, it became apparent that Registration objects were expected to be recreated, but are currently being re-used (https://gitlab.science.ru.nl/thalia/concrexit/issues/484#note_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?).