Registration can be disabled for the entire instance or blocked from certain
IPs (e.g. botnets trying to flood mailboxes with registration mails from all
online services they can find)

All travelynx and IRIS timestamps are Europe/Berlin. And the DateTime objects
know that they are Europe/Berlin. However, when inserting into a timestamp with
time zone column, Mojo::Pg or one of the backend DBI/DBD libraries does not
respect the time zone. So, UTC+1 or UTC+2 timestamps are treated as UTC+0,
resulting in major mayhem.

(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)