Deletion of users

[inyono]

Automate this:

• Set user id to id 4 (fake combined user for deleted users) for all relevant tables (e.g. notifications are definitely not needed)
• Delete user from user
• Review where we could delete more (e.g. which events are not needed anymore, comments, ...?). E.g. look for usages of events, e.g. trash bin

Handle:

• When rendering username, maybe handle id 4 specifically
• Profile for id 4 (e.g. no activity log / stats), info that this is a deleted user
• When user does not exist anymore, redirect profile to /4 since deleted.

[kulla]

We track this already in https://github.com/serlo/api.serlo.org/issues/298