Shifts/Reports: `locked` field migration

So far the `locked` field signals whether a month is already locked and the report is archieved in the time vault.

However i have the impression that we should change that so that

- [ ] Migrate Field `locked` from `Shift` to `Report` (Includes data migration *sigh*)

- [ ] Refactor the validation of `ShiftSerializer` to check whether the months `Report` instance is already locked

- [ ] Refactor `lock_shifts()` Endpoint of `ContractViewSet` to set `locked` status on Report rather than shifts

- [ ] Refactor all Querries that access `locked` field of Shifts to now access `locked` of Report