(yes, there are actually spambots flooding people with unsolicited
registration e-mails)

This ensures unambiguous checkins for ring lines. See
https://github.com/Traewelling/traewelling/commit/eaf975a3337a151c4759428747342ce49eadea47

TODO: handle CET -> CEST transition (there's a day each year where 2am does not
exist and DateTime does not like it)

HTTP 406 indicates that a user has not accepted the ToS / privacy policy

Issue has been fixed