Bug 35027: Add 'hold' to patron activity triggers

Bug 35027: Add 'hold' to patron activity triggers

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

Test plan
1) Select 'Placing a hold on an item' in the
   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>