Bug 35030: Add 'article' to patron activity triggers

Bug 35030: Add 'article' to patron activity triggers

This patch adds 'article' to the list of triggers available for
tracking patron activity.

Test plan
1) Select 'Placing an article request
   TrackPatronLastActivityTriggers system preference
2) As a staff member, place a hold on any item for a test user
3) Confirm that the borrowers.lastseen field is updated for that
   test borrower

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>