Refactoring MongoResource base class
Implemented validation (format, fields, unicity) over Activity Resource and POST
MongoResource send signal when one document has been inserted into the collection
Remove this fields, isn't used in queries or similar
Implemented registered filters over MongoResource and MongoUserResource
Starting refactoring answer resource and make functional kq resource for frontend developers
Adapt classroom to manage properly the new activity structure
Refactor answer collection and resouce
Merge branch 'master' into mongo_refacts

Conflicts:
	moocng/api/resources.py
Bugfix when loading activity from backend
Adapt answer manipulation to the new api

The API has changed due to the optimizations performed to the mongodb
Merge branch 'master' into mongo_refacts
Fix signals from MongoDB Resources
Refactoring mark per student calculation
Refact get_user in OptionResource
Refact KnowledgeQuantum mongo access
Refactoring mongo access in marks and Teacheradmin stats views
Add migration to optimize activity structure in mongodb
Use a JS script to make the migration, it's way faster
Bugfixes in the activity optimization migration