Bug 19323: subscription edit permission issue

Bug 19323: subscription edit permission issue

If a librarian has edit_subscription but not create_subscription :
When trying to edit a subscription, after saving permission is denied.

This is because permissions in serials/subscription-add.pl depends on arg 'op' and on edit this arg starts with 'modify' but changes to 'modsubscription' when saving.

Test plan :
- Create a user with staff access
- Define its permissions on serials : only edit_subscription
- Edit a subscription
- Click 'Next'
- Click 'Test prediction pattern'
- Click 'Save subscription'
=> Without patch you get to page serials/subscription-add.pl with permission denied
=> With patch subscription is saved and you get to subscription details page

Signed-off-by: Caroline Cyr La Rose <caroline.cyr-la-rose@inlibro.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>