Bug 10855: interface configuration for adding new fields to subscriptions

Bug 10855: interface configuration for adding new fields to subscriptions

This patch adds an interface configuration for adding fields to
subscriptions.

Test plan:
- go to serials/serials-home.pl
- click on the "Add subscription fields" link on the left of the screen
- try to add/update/delete new fields
You are not abble to add a field with an authorised value category and a
marcfield. Columns are sortable
- for the rest, you should have, at least:
  * a new field named af1, sortable, linked to a category
  * a new field named af2, sortable, linked to a marcfield
  * a new field named af3, not sortable

Signed-off-by: Brendan Gallagher <brendan@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>