Update attendance registers before printing attendance sheet

This patch only updates attendance registers of events which haven't taken place yet ([lines 396-399](https://github.com/elitalon/academic/blob/dad4cb8cbc5174003aad74e6ed2854721a0e170e/app/controllers/attendance_registers_controller.php#L396-399)). Here is why:

Once an event is past and its attendance sheet has been printed, a teacher could edit the attendance registers manually (see below). Thus, there is no way to know if mismatched records are there for some reason (e.g. a student relocated from another group) or they're just wrong.

Past activities with wrong attendance registers (or wrong attendance sheets) should be edited and fixed manually at `/attendance_registers/view_my_registers/<id>`. This form can be reached from a link in the _"Cursos"_ section called _"Editar asistencia de estudiantes"_.

I have some doubts regarding teachers editing the attendance sheets __before__ printing them. If that's possible the solution is more complicated: it involves modifying at least two more controllers.