Split up models.py

The core/models.py is getting quite big and hard to navigate.

Proposed different layout:

-- core
  |-- models
    |-- __init__.py
    |-- Feedback.py
    |-- Submission.py
     

The core/models/__init__.py re-exports all the Models from the core/models folder so there should be no need to change any of the imports in other files.

added Backend Refactor labels

changed milestone to %Deadline 18.05

Is done in e1cc9705

closed