Bug 35032: Remove the use of "onclick" from Koha to MARC mapping template

Bug 35032: Remove the use of "onclick" from Koha to MARC mapping template

This patch eliminates two instances of "onclick" from the Koha to MARC
mapping template, moving these event handlers into JS in the footer.

To test, apply the patch and go to Administration -> Koha to MARC
mapping.

- Click "Add" next to a Koha field.
  - You should get a JS popup asking for entry of a MARC tag. Confirm
    that this process completes correctly.
- Test a "Remove" link too. It should work to remove mapping from a Koha
  field.
- Test the "Cancel" choice in both the "Add" and "Remove" cases to
  confirm that the operation is cancelled.

Signed-off-by: Andrew  Fuerste-Henry <andrewfh@dubcolib.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>