Bug 34478: Corrections to add and delete of OAI sets

Bug 34478: Corrections to add and delete of OAI sets

This patch makes two changes: The first changes the name of the op value
matched in the script when editing a set. The "mod" step is a GET
operation to load the edit form.

The second change is a workaround for the fact that a submit
button looks bad in a Bootstrap dropdown. The patch creates a hidden
form for deletion operations. Clicking a "delete" link in a dropdown
fills the hidden form with the OAI set id to be deleted and submits it.

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