Anonymize old reservations

[smessie]

For the GDPR, we are not allowed to keep reservations longer than necessary to organize the event. Since we want to keep statistics, we have to anonymize the reservations every year. This can be done e.g. by letting a dummy user own all reservations, or by setting the user to null (we have to be careful with the constraint).

A script should be written for this.

Specific idea: for each HOI we want to have a dummy user so we can change the users of reservations to these dummy users.
By having a dummy user per HOI, we can still keep statistics of which HOI students have reservations at certain locations. However, we do lose the ability to see how often students reserve/cancel/.... (so things at the level of individual students are dropped) but this is a trade-off between anonymizing and keeping enough data for statistics. In the past, statistics at the level of individual students might be less useful or interesting anyway so this trade-off can be made.

Then provide a script that moves user_ids of reservations with those HOI dummy users to anonymize, ideally just before the start of a new exam period so that we always have all the data from the last exam period (e.g. to draw statistics).