With these changes, we allow the contract line start date to be computed
using the order confirmation date. When the product is configured with
any of the options set in contract_start_date_method other than manual,
the start date will be calculated based on the established date and the
selected period.
Additionally, we can force the month in which we will work in case the
frequency is yearly, quarterly, or semesterly.
Is not added support for daily, weekly or monthlylastday in this commit.
monthlylastday is (almost) not a special case anymore \o/.
montlylastday is simply a montly period where the
periods are aligned on month boundaries.
The last bit of special casing is that postpaid generates
invoice the day after the last dasy of the period, except
for monthlylastday where the invoice is generated on the
last day of the period. This last exception will disappear
when we put the offset under user control.
This is a breaking change because the post-paid/pre-paid
mode becomes relevant for monthlylastday invoicing.
The field becomes visible in the UI. Code that generate
monthlylastday contract lines must now correctly set
the pre-paid/post-paid mode too. Some tests have had
to be adapted to reflect that.
