Bug 18545 [Follow-up] Remove use of onclick from OPAC Cart

Bug 18545 [Follow-up] Remove use of onclick from OPAC Cart

This patch makes a few corrections:

- Use 'e.preventDefault()' instead of 'return false' in changed
  sections.
- Modify the event handler on checkboxes to successfully handle the
  un-check action as well as the check action. (QA comment 5).

To test, follow the original test plan.

Signed-off-by: Aleisha Amohia <aleishaamohia@hotmail.com>
Signed-off-by: Jesse Maseto <jesse@bywatersolutions.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>