services.redshift: Enable auto-restart.

The redshift service stops working after some time (the program exits
after some hours/days). It looks like these exits are related to calls
to xrandr (for multiple displays) or suspend-to-ram.

Signed-off-by: Moritz Ulrich <moritz@tarn-vedra.de>
services.redshift: Start after display-manager.service.

Signed-off-by: Moritz Ulrich <moritz@tarn-vedra.de>