Update contract end date with locked shifts raises modified minutes error

#### Observation

Users try to change their contract's end date routinely while locked shifts exist.
They receive and error `"The minutes of a contract with locked shifts is not allowed to modify."`

The error is raised here: https://github.com/ClockGU/clock-backend/blob/develop/api/serializers.py#L235-L241

#### Steps to reproduce

1. Create a contract starting a month or so in the past
2. Create some shifts in the 1st month
3. Lock and submit
4. Open the contract to edit and set a new end date
5. Save - the error will appear