For the same reasons as with expenses[0], users are no longer expected
to manually set invoice status, and is now linked to their collections.
In this case, however, we had to remove the ‘sent’ and ‘unpaid’ status
options, because these _should_ only be set manually, as there is no
way for the application to know when to set them. Thus, there could
be inconsistencies, like invoices set to ‘unpaid’ when they actually
have collections, or invoices that were ‘sent’, then transitioned to
‘partial’/‘paid’ due to a collection, but then reset to ‘created’ if the
collection was deleted.
[0]: ac0143b2b0
This is the same as a payment, but the user is the payee instead of the
payer.
I used a different relation than payment because i do not know any other
way to encode the constraint that only invoices can have a collection,
while expenses have only payments.
Besides the name and the fact that they are related to invoices, a
collection is pretty much the same as a payment.
jordi fita mas