Bug 10794: fix sorting on billing date column in invoices table

Bug 10794: fix sorting on billing date column in invoices table

This patch adds the sorting by title string option to the table of
invoices. This allows column data to be sorted based on the
ISO-formatted date rather than the formatted-for-display date. A "blank"
(0000-00-00) date is added to cells which contain no date data.

To test, view the Acquisitions Invoices page (acqui/invoices.pl) and
confirm that the "Billing date" column is sorted correctly regardless of
the dateformat system preference.

Signed-off-by: Srdjan <srdjan@catalyst.net.nz>
Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Template only change, passes all tests and QA script.
Sorting of the billing column now works as expected.

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