Related to #828
- Enable web UI for API/Terraform schedules to add overrides
- Refactor backend to add a flag toggling between web-based and
iCal-based overrides (these options are mutually exclusive)
Also updated read-only tooltips (related to #1483)
* Centralize timezone validation into one spot + add serializer validation
for schedules and oncall shifts (both public and internal API)
* add engine-manage make command