Bug 28142: Accessibility: OPAC cart/basket checkboxes are not labelled

Bug 28142: Accessibility: OPAC cart/basket checkboxes are not labelled

This patch adds labels to the checkboxes on the "Your cart" page to
improve accessibility for users of screen readers. The labels are
hidden from display because the item titles provide enough visual
context.

Test Plan:

1. Go to the OPAC
2. Search for some items and add them to your cart
3. Inspect the page html and confirm the checkboxes are not labelled
4. Select the "More Details" page and confirm these labels are missing too
5. Apply the patch
6. Inspect the checkboxes on the "Brief display" and "more details" views
7. Confirm that the checkboxes have meaningful labels and that the visual
display of the pages has not changed.

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
JD amended patch: fix indentation

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>