Consolidate the logic for the list of courses an admin will see in the calendar.

They will see all courses if calendar_admincanseeall is enabled.

They will see only courses they are enrolled in of calendar_admincanseeall is not enabled.

They will also see the current course, if they are admin and it is not already in the list.

This applies to the course selector and the create and update event forms.

The calendar link in the navigation block has also been changed to link to the current course calendar instead of the global
one.