27 - Gelockte Slots und Assignments

added a new class in backend/models.py AcceptedApplications to model the relation between an assignment and an application for the accepted applications, to be able to store whether an application is locked added a new attribute (finalized_slot) to the Assignment class in backend/models.py to store whether a slot is locked added a button on the backend/assignment.html for every displayed slot as well as for all displayed applications in any slot added two new functions to backend/assignment_page.py to handle the change of the finalization value of either a slot or an application changed the export to support the feature of locked slots and assignments, therefore adding two helper-functions to backend/export_applications_and_assignments_page.py changed the automated tests in tests.py to fit the additional return parameters of some functions

closes #27 (closed)