Moved group selection to the year page and simplified the query of the subject view. Now it does everything through the django ORM API instead of implementing the distinct manually (which was kinda hacky).

Modified the templates to adapt to those changes.
Disable duplicate classes entries
Changed Class model name to Lesson, since it is much easier to name objects after it.
Added tests that check that the model contrains regarding insertion of duplicate elements work.
Lesson representation shows subject
Better handling of object duplicates on the test case
Class parser parses optional subjects too
Renamed classparser to lessonparser to reflect the refactoring of the models.py