SSP-2725 Limit /caseload API results to operational data

So now the caseload tab and caseload reassign will only ever return
students with records in person. Students only present in
external_person will be excluded. You can still get your entire
caseload, internal and external, via directory search.

This does not change the behavior of the caseload tab b/c that
tab always implicitly limited results to operational records
by requiring a program status. It does change the behavior of
bulk caseload reassign which previously returned internal
and external students. But the latter were pointless b/c we
can't reassign an external record.