Bug 15981 - Serials frequencies can be deleted without warning

 Bug 15981 - Serials frequencies can be deleted without warning

A JavaScript alert should ask the user to confirm deletion of a serial
frequency. This patch adds this to the template.

To test, apply the patch and go to Serials -> Manage frequencies

- In the list of frequencies, click any "Delete" link.
- Verify that a JavaScript alert is triggered.
- Test that cancelling the alert cancels the deletion operation.
- Test that confirming the alert allows the deletion to complete.

Signed-off-by: Nicole C Engard <nengard@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Brendan A Gallagher <brendan@bywatersolutions.com>