Allow PD migrator tool to migrate on-call shifts when migrating
schedules (currently it migrates schedules using PD ICal file):
https://github.com/grafana/oncall/issues/1283.
This PR will allow to select the mode of schedule migration via
`SCHEDULE_MIGRATION_MODE_WEB` env variable (`ical` or `web`). Due to
differences in the scheduling systems of PD and OnCall, it's not always
possible to migrate shifts automatically (migration plan will show any
schedules and layers that can't be migrated).
PD rotations that will be possible to migrate:
- Any rotation without restrictions ("restriction" is a PD term for
describing active periods for rotation)
- Daily rotations with daily restrictions
- Weekly rotations with weekly restrictions
- Some weekly rotations with daily restrictions
- Some daily rotations with weekly restrictions
There will be a separate PR to update the
[instruction](https://github.com/grafana/oncall/tree/dev/tools/pagerduty-migrator#readme)
since this one is pretty huge already.
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