Closes #44

Work in progress. Only status is documented at the moment.

Select top station on work days (Mo .. Fr) with arrival < 13:00 or
departure >= 13:00.

order is ascending by default, resulting in flop instead of top destinations.

Until now, tripIDs were cached based on station and departure timestamp.
These are identical for any two trains departing at the same time at the same
station, leading to one of those getting being assigned a wrong tripID.

From now on, only the JSON reported by transport.rest is cached -- tripIDs
are always recomputed based on it.

Closes #40

Previously, for each destination, up to two departures between now and
now + 40 minutes were shown.

When performing a fresh checkin (i.e., the has not logged a journey in the
past few minutes and is not currently checked in), the departure list now
additionally contains all departures between now - 5 minutes and now.

When checking in from a train (i.e., the user has recently checked out at
the station or is still checked in), it additionally contains all departures
between arrival time and now.

Closes #33

Squashed commit of the following:

commit 3dd25e35bb8c9d238e64300494b8c4e05e494bba
Author: Daniel Friesel <derf@finalrewind.org>
Date:   Mon Apr 6 10:02:57 2020 +0200

    hm

commit 04159061ed7932e2ee4ceca7af9d43a3fca034c5
Author: Daniel Friesel <derf@finalrewind.org>
Date:   Mon Apr 6 09:56:19 2020 +0200

    no sudo

commit 1d16ec10faa807672433cbab11acd50eee9c8a6f
Author: Daniel Friesel <derf@finalrewind.org>
Date:   Mon Apr 6 09:51:54 2020 +0200

    Switch from Travis CI to GitHub Actions