Make recurrence mechanism on contract line and some other refactoring
[FIX] - Keep contract_cron on account_analytic_account model
contract_cron defined with no_update option.
Changing it, will cause issue to past version installation.
[IMP] - Fix recurring_next_date default value
recurring_next_date should have start_date as default value in prepaid policy
and start_date + invoicing_interval if postpaid
[FIX] - Fix test check no journal
[IMP] - Return created invoices on recurring_create_invoice
[IMP] - Specific process to compute recurring_next_date for monthly-last-day
fixes: #198
[ADD] - Add Post-migration script to bring recurrence info from contract to contract lines
[ADD] - Add search filter based on date_end and recurring_next_date
- not_finished filter in contract search view
- finished filter in contract search view
- Next Invoice group by in contract search view
[ADD] - Add unit tests
- cases to compute first recurring next date
- contract recurring_next_date
- contract date_end
[IMP] - Improve Unit tests
