* fix: use DST-friendly tz calculation for TimeSlot end times
Around DST changes, localize(time + duration).astimezone() is not
the same as localize(time).astimezone() + duration. The latter allows
tz-aware times to do their magic and give correct clock times.
* fix: refactor/correct calculation of week-view event start/end times
There were bugs in the timestamp calculations around DST switchover.
This corrects those and tries to clarify and improve documentation in
the relevant code.
