[Contracts] Contract period from rejected contracts cannot be reused

[MarG66]

Preconditions:
The user must be logged in on the "Lista contracte" tab.
At least one template must be available and the NGO must have at least one active volunteer.

Steps to Reproduce:

1. Generate a contract
2. Reject said contract, either via mobile or web app
3. Generate another contract with the same contract period as the one from step 1

Expected Results:
3. The second contract is created.

Actual Results:
3 The second contract cannot be created because the contract period is seen as duplicate.

Note:
If a contract is rejected by either side(due to incorrect id data or other issues), when the contract is re-created, the web admins should be able to use the same contract period as the one used in the rejected contract.

[MarG66]

@dragos1195 Testing failed, bug can still be reproduced.