CONTRIB-8227 Memory overflow

[DavidGlasgow]

When editing a scheduler slot in teacher view, the list of student appointments is presented to the teacher using formslib repeat_elements.

The element being replicated, at minimum, contains the following mform elements:

appointhead header
student group, which contains:
studentid searchableselector
appointid hidden
attended checkbox with label "seen"

Additionally, scheduler may add the following form elements based on configuration of the scheduler:
grade select with label "grade" to the above group
appointmentnote editor
teachernote editor

Then all appointments have
deletestudent checkbox

The header element includes the "{no}" wildcard which injects a numeric count into the Appointment header, e.g. "Appointment 1, Appointment 2 etc.."
formslib repeatelements always starts this count at zero and doesn't support paging of this numeric out of the box.

studentid as searchableselector with many available students on the course of the scheduler, can dramatically increase the RAM usage per appointment.

In testing, a representative course without grading, appointment or teacher notes, with a single slot with 89 appointments, on a course with 375 participants, generated a peak RAM of 1.3 GB, or about ~15 MB per appointment.

So we have two combining forces: The number of appointments in a single slot, multiplied by the number of participants in the course.

In my testing, changing studentid from searchableselector to autocomplete reduces the RAM footprint for each appointment that is repeated to ~2 MB to the RAM usage on the server, or for 89 appointments, 130 MB, just higher than a commonly usage php_memory_limit value of 128 MB.