Bug 23153: In framework management action subfields must not go directly to edition

Bug 23153: In framework management action subfields must not go directly to edition

Since 18.11, when viewing a framework field clicking on Actions >
Subfields goes to subfields edition instead of subfields table view.

This is because link contains op=add_form.
This is is also missing /cgi-bin/koha/admin.

Test plan :
1) Go to Administration > MARC bibliographic framework
2) On a line click Actions > MARC structure
3) On a line click Actions > Subfields
4) You see subfields table
5) Click on Edit subfields
6) You see subfields edition tabs

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>