Before this change, a diff ical check (which happens with frequency with
imported ical), particularly with overrides in an API/terraform schedule
would trigger unexpected slack notifications because the prev vs current
ical comparison will flag a diff, but when comparing current and
previous shifts, `current_shifts` will have the shift in progress while
the `prev_shifts` calculated from the overrides-only diff will most of
the time be empty (unless you set/change an override at current time).
Simplified the checks to always compare previous current shifts (ie. the
ones in the schedule from the DB) vs the recalculated ones using the
(refreshed) ical data from the schedule.
5d383c7d1d