Linked courses in periods with courses in library

when editing either a course in the library or a course in a period
their counterparts should automatically update.

-This broke the save function partially
-Added a catch for when save file wasn't found, since i've been to lazy
to figure out where to put it.
-To remove courses from the library once they've been added to the
selectedPeriod we need an unique identifier for all courses. (also
needed for the database so yay)