added module hr_contract_multi_jobs

This module removes the job_id field on contract view and adds a new field contract_job_ids. contract_job is a relationnal object between contracts and job positions.

A many2many field is not adequate because, it does not allow to specify information about each job for a contract. Example, an other module implements hourly rates. Many contracts could have the same job position but with different rates. In this case, we can add a field "hourly_rate" in the contract_job model.
@pedrobaeza Yes, I check that with the constraint _check_one_main_job.