show select past departures in checkin suggestion list

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