Added tests that check that Lessons are deleted and inserted when an external calendar is updated.

Also added code (still not finished nor tested) that will update the .ics files that contain Subjects which have been updated.
uploaded file needed for the test
The update method of the LessonParser command returns a QuerySet of the updated DegreeSubjects (if any). There is also a test that proves this functionality is working.

Additionally, there is a new method, update_calendars(), that takes a QuerySet of all the modified DegreeSubjects, selects any calendar that has matching entries and redoes them.

This last new method needs testing.
New test to check DegreeSubject method lessons()