LP#1986479: fix lookup of user locale during A/T event processing

LP#1986479: fix lookup of user locale during A/T event processing

This patch fixes an issue with the new user-locale-sensitive
alternate Action Trigger template selection that supports letting
patrons receive notifications in their preferred language. In
particular, it corrects an assumption that the event target is
the user itself. With the patch, the the event definition's
context user path is consulted to identify the relevant user.

To test
-------
[1] Set up a patron with an email address and check out an item.
[2] Use the Quick Receipt drop-down to attempt an email checkout
    receipt.
[3] No email will be generated an the event will fail. The logs
    should contain something like

    Can't use an undefined value as a HASH reference at
    /usr/local/share/perl/5.28.1/OpenILS/Application/Trigger/Event.pm
    line 518.
[4] Apply the patch and repeat steps 1 and 2. This time, the email
    should be sent (or, at least, the A/T event will be successfully
    processed).

Signed-off-by: Galen Charlton <gmc@equinoxOLI.org>