Tutor policy decisions should be per-subject

[Matt Giuca]

There are currently two policy decisions which can be configured in ivle.conf: tutors_can_enrol_students and tutors_can_edit_worksheets. These decisions are really a factor of how many tutors there are in the subject and how trustworthy they are, which should be determined on a subject-by-subject basis. Therefore, they should be moved into the database as decisions made by the subject coordinator. The configurations in ivle.conf should just influence the default settings for these decisions.

Alternatively, individual tutors could be granted this permission (which would enable, for example, a head tutor to have powers while the other tutors do not). Or, simply adding a head tutor role which has this power while the ordinary tutor role does not.