This fixes scenario described
[here](https://github.com/grafana/oncall/issues/2118#issuecomment-1580499754).
When a rotation is setup in UTC+1, and the shift starts at 00:00 with
Monday as active day and a weekly frequency, the values are translated
to UTC when submitting to the backend, so the shift data becomes
something like: shift starting at 23:00 on Sunday, but since week_start
is on Monday, the "first event" in the week belongs to the "previous
week". This can be addressed by moving the week_start, so a weekly shift
that was starting on a Monday but in UTC tz starts on Sunday,
"translates" to a UTC week_start on Sunday:
(this is with the proposed changes; otherwise you get the same issue
linked above where the first event in the week is assigned to the other
user group).
About selected week days changed when editing a rotation, see inline
comment (related to
[this](https://github.com/grafana/oncall/issues/1322#issuecomment-1521787786))
6e5d417967