Ability to mark a module for as tutoring instead of reading

[RezwanArefin01]

Have a way to mark the modules that the user is not reading but tutoring.

A possible way to do this is to have an additional state for the hide icon, which will make the corresponding slots in the timetable have dotted border with transparent background. However, this solution will not work if the user is tutoring more than one slot.

Related: #3339, as the user may want to hide some slots (Lecture for example) he will not be tutoring. Also #3404.

[ravern]

Similar issue being tracked in #2040.