remove copy across study across assessment

[shapiromatron]

Remove old copy_across_assessments code - it was untested and likely had many errors. If rewritten in the future, this may be useful as a resources. However:

• instead of putting into the models.py, move to a services style structure?
• consider clone(parent=...) as a different structure?
• build some sort of non-dict data structure to properly store the state changes
• unit tests for each component!

[munnsmunns]

Looks good to merge!