Lookup existing attendances by only session and teacher, then set enr…

[aoby]

…ollment after, to reduce chance of RecordNotUnique error

Errors in the wild:
https://app.honeybadger.io/projects/3240/faults/33792540
https://app.honeybadger.io/projects/3240/faults/33760831

ActiveRecord::RecordNotUnique: Mysql2::Error: Duplicate entry... for key 'index_pd_attendances_on_pd_session_id_and_teacher_id': INSERT INTO `pd_attendances`

attendance.rb  63  block in find_restore_or_create_by!(...)
attendance.rb  61  find_restore_or_create_by!(...)
workshop_attendance_controller.rb  67  create(...)
...

I'm not actually sure what's causing this error, but I suspect maybe there's an existing (deleted) entry with either a different enrollment or no enrollment, and this attempt to recreate it with a new enrollment id is conflicting. In both cases, I logged in and was able to set attendance successfully in the UI.

This PR separates the lookup (now just teacher and session, the unique fields for the index) from updating the enrollment. If my hunch is wrong, this shouldn't cause any problems. It will still result in the same outcome, and we were already making a second SQL call to update the marked_by_user.