Bug 10855: Search subscriptions by additional fields

Bug 10855: Search subscriptions by additional fields

Now you will abble to search subscriptions by additional fields.
The additional field values will be displayed in the table results.

Test plan:
- go on the subscriptions advanced search page
  (serials/serials-search.pl).
- verify all searchable additional fields are displayed on the form.
- combine 1 or more values and verify results are consistent.
- verify the values are displayed in new columns of the table.
- for field linked to an authorised value category, the description is
  displayed (not the code).

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