Bug 17530: (QA follow-up) Add pref ArticleRequestsLinkControl

Bug 17530: (QA follow-up) Add pref ArticleRequestsLinkControl

As requested by RM on Bugzilla comment53.

This pref allows you to always display the link on search results.
Which is the previous behavior ;) It will raise false positives, whereas
the algorithm may result in false positives or negatives (depending on
default item type and circulation rules).

When upgrading the pref is set to always (current behavior), new installs
get the new calc value using the algorithm in may_article_request.

Test plan:
Run t/db_dependent/Koha/IssuingRules/guess_article_requestable_itemtypes.t
Run t/db_dependent/ArticleRequests.t

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>