Closes #143

This is in preparation for supporting multiple HAFAS backends, and possibly
EFA and RIS::Journeys.

Replaces the theme switcher in the nav bar with a less intrusive link at the
bottom.

Closes #125

This way, consecutive history entries have matching origin/destination pairs,
and both the outer list and the inner origin/destination pair are sorted
by descending date.

(cherry picked from commit 8b3ff460b5845c4ec86466715fc054705b635c20)
Signed-off-by: Birte Kristina Friesel <derf@finalrewind.org>

(cherry picked from commit 5fe0b4ac921c338ae642f88f2f44804d285f4691)
Signed-off-by: Birte Kristina Friesel <derf@finalrewind.org>

(cherry picked from commit 20e8fd39851146eb4fb3cc8cb1f7ba7fc4086d8d)
Signed-off-by: Birte Kristina Friesel <derf@finalrewind.org>