Bug 21467: Allow several receipts for a given subscription

Bug 21467: Allow several receipts for a given subscription

When a new order is created from a subscription the quantity is set to 1
and cannot be modified.
The idea was to order 1 subscription.
This behavior leads to a limitation: it is not possible to mark a
receipt as partially received. However it is how it works in real life,
the vendors send invoices throughout the year. The number of items can
also be changed.

The idea is be to rethink the "quantity" value for an order created
from a subscription and use it to track the number of invoices already
paid.

FIXME: This approach will not cohabit with standing orders.
This patch is a first draft to get feedback on the idea.

FIXME: What about cancelled orders?

Test plan:
- Create a new order from a subscription
- Enter the number of items you think you will receive for this
subscription (for instance 1 per month: 12)
- Close the basket
- Receive 3 items (a trimester) and create a specific invoice for this
receipt. Note that the price are per unit.
If you want to receive items with different prices you should make
split the receipt
- Receive more items. This time you will notice that the previous order
will be displayed on the "order receive" under a new block "Receipt
history for this subscription"
- Note that the "Quantity to receive" has been decrease by the number of
items you previously received
- Also you can notice that this "Quantity to receive" can be modified.
Indeed it can happen that the number of items to receive changed during
the year
- Go to the detail of the subscription and notice that the orders have
been grouped by "parent ordernumber"
- Continue to receive items until all have been received

Sponsored-by: BULAC - http://www.bulac.fr/
Signed-off-by: Séverine QUEUNE <severine.queune@bulac.fr>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>