Bug 7288: add filter on subscriptions in the invoices table

This patch adds a checkbox "Show only subscriptions" in the invoices
table.

If this checkbox is checked, only invoices that contain at least one
order linked to a subscription are displayed.

To test:
- Test in a database with multiple existing invoices
- Create an order from a subscription, close basket, receive
- Test that the result table of the invoice search shows
  the new checkbox and that it works correctly

Signed-off-by: Paola Rossi <paola.rossi@cineca.it>
Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Bug 7288: (follow-up) set a boolean if the invoice if linked to subscriptions

If an invoice is linked to subscription, we need to set a boolean to
true in order to filter them in the interface.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Bug 7288: (follow-up) various fixes

- Fix syntax error in supplied test
- remove subscriptionid as a field returned by GetInvoices(), as
  the is_linked_to_subscriptions Boolean takes its place.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Bug 7288: (follow-up) add unit test for is_linked_to_subscriptions flag

This adds a direct unit test of the is_linked_to_subscriptions flag
and corrects a typo in the previously-submitted test.

To test:

[1] Verify that prove -v t/db_dependent/Acquisition/OrderFromSubscription.t
    passes.
[2] Verify that prove -v t/db_dependent/Acquisition/Invoices.t
    passes.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>