This is a breaking change insofar as that traewelling support is no longer
provided automatically, but must be enabled by providing a traewelling.de
application ID and secret in travelynx.conf. However, as traewelling.de
password login is deprecated and wil soon be disabled, travelynx would break
either way. So we might or might not see travelynx 2.0.0 in the next days.

Automatic token refresh is still todo, but that was the case for password
login as well.

Closes #64

Closes #4

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)

dockercompose

This avoids having to specify secrets in the environment, where they can leak
easily.