Allow linking a Träwelling account, auto-sync Träwelling→travelynx

travelynx→Träwelling is still work-in-progress

Squashed commit of the following:

commit 97faa6e2e6c8d20fba30f2d0f6e78187ceeb72e6
Author: Daniel Friesel <derf@finalrewind.org>
Date:   Wed Sep 30 18:50:05 2020 +0200

    improve traewelling log and tx handling

commit 487d7dd728b9d45b731bdc7098cf3358ea2e206e
Author: Daniel Friesel <derf@finalrewind.org>
Date:   Wed Sep 30 18:02:41 2020 +0200

    add missing traewelling template

commit 0148da2f48d9a52dcddc0ab81f83d8f8ac3062ab
Author: Daniel Friesel <derf@finalrewind.org>
Date:   Wed Sep 30 18:02:35 2020 +0200

    improve traewelling pull sync

commit 4861a9750f9f2d7621043361d0af6b0a8869a0df
Author: Daniel Friesel <derf@finalrewind.org>
Date:   Tue Sep 29 22:14:24 2020 +0200

    wip checkin from traewelling

commit f6aeb6f06998a2a7a80f63a7b1b688b1a26b66bd
Author: Daniel Friesel <derf@finalrewind.org>
Date:   Tue Sep 29 18:37:53 2020 +0200

    refactor traewelling integration. login and logout are less of a hack now.

    checkin and checkout are not supported at the moment.