Add calculated field "current tariff" in contract

• Add calculated field "current tariff" in contract In every contract we have contract lines. Within them, there is the main contract line (tariff) and some additional services. We add "Current tariff", a computed field containing the contract line.

• Add current_tariff_contract_line in view

• Add test to check contracts without active tariffs test_route_contract_without_active_tariff: Check that contract.current_tariff is False with a contract whose main contract is not active, since its date_start has not arrived. Check also that additional contract lines don't count.