LP#1879983: A/T components of Curbside

LP#1879983: A/T components of Curbside

This commit adds a reactor and validator for automating parts of the
basic Curbside workflows:

* Curbside validator: check whether curbside is enabled at the org unit
  applicable an A/T event's target, which could be a user, org unit,
  curbside appointment, or hold request.
* CurbsideSlot reactor: Creates a curbside appointment slot at the hold
  pickup library when a hold becomes ready for pickup, if one does not
  exist. This is meant to be triggered by the hold.available hook.
  Appointments created by this reactor do not have an appointment time
  set, as that is meant to be supplied by the patron or a staff member
  acting on behalf of the patron.

In addition to Mike Rylander, significant contributions to this
patch were made by Jason Boyer.

Signed-off-by: Mike Rylander <mrylander@gmail.com>
Signed-off-by: Jason Boyer <jboyer@equinoxinitiative.org>
Signed-off-by: Galen Charlton <gmc@equinoxinitiative.org>