Add/remove students from a reallocation group (legacy PATCH /reports/reallocation/students)

Requires permission: ReportAdmins

Removes the supplied entries and inserts a new entry per added pupil, seeding each new entry's mark-grades from the group's existing grade fields. Port of the legacy RPT_RA_MarkGradesForStep / RPT_RA_DeleteEntry / RPT_InsertEntry / RPT_InsertMarkGrade chain.