Bug 20563: Allow display of requested partners

Bug 20563: Allow display of requested partners

This patch adds the display of requested partner email addresses when an
ILL backend provides the ability to send requests to partners. Partner
email addresses are displayed in the illlist and illview displays, they
are also included in the 'illrequests' API response.

* api/v1/swagger/paths/illrequests.json:
  - Add 'requested_partners' as an 'embed' enum
* Koha/Illrequest.pm:
  - Add 'requested_partners' accessor calling optional backend 'get_requested_partners' method.
  - Store requested partners upon email send, calling optional backend 'set_requested_partners' method.
  - Add 'requested_parners' embed to overloaded TO_JSON method.
* koha-tmpl/intranet-tmpl/prog/en/modules/ill/ill-requests.tt:
  - Add 'requested_partners' embed to illrequests API call
  - Add render function for "Additional status" datatables column
  - Add display of requested partner email addresses to illlist table
  - Add display of requested partner email addresses to illview display

To test:
1) Enable Interlibrary loans
2) Add a backend that supports sending requests to partners, e.g.
FreeForm
3) Set up at least one partner
4) Create an ILL request
5) Send request to partner(s)
6) Observe partner(s) email address(es) are displayed in "View ILL
requests" view
7) Observe partner(s) email address(es) are displayed in "Manage ILL
request" view

Signed-off-by: mmg@interleaf.ie
https://bugs.koha-community.org/show_bug.cgi?id=20653

Bug 20563: (follow-up) Fix requested partners

As per:
https://bugs.koha-community.org/bugzilla3/show_bug.cgi?id=20563#c10

Thanks for the suggestion on merging the "Status" and "Additional status" columns, looks much better!

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>