Bug 10855: New fields on adding/updating a subscription

Bug 10855: New fields on adding/updating a subscription

This patch adds the ability to add values for additional fields when
adding/updating a subscription.

Test plan (test the following on adding a new subscription and on
editing an existent one):
- add/edit a subscription.
- verify all additional fields appears into the "Additional fields"
  block.
- fill a value for af1 and af3.
- fill others values if you are adding a new subscription.
- save.
- verify values appears on the detail of the subscription page.
- verify the af2 field is automatically filled with the specified
  marc field of the notice.

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