Move students from attendance_template to attendance_schedule

[FherStk]

This allows:

• Different students per scheduled session, so no need for multiple templates for the same groups, teachers and subjects.
• Direct relation between resource.calendar.attendance and attendance_schedule.
• Creation and archival of templates will be handled by the student's schedule, so no manual creation or archival will be allowed anymore (since we handle employees attendance and schedules, the attendance_template is not the top entity anymore).