Added permission's control to modify transcripts, based on period and year of
the requested transcript and on the "transcript.modification.open" global
configuration. If transcript's period/year are equal to the current period/year
and transcript's modification period is open the professor may modify the transcript.
Also, this controller doesn't check if the transcript belongs or not to
the logged-in professor. To do this is used "hasPermissionToView" function.
Signed-off-by: Bruno Nocera Zanette <email@example.com>