Fix for issue #2358 gracefully handle exercise interactions for admin users

Fix for issue #2358.

Admin user can take exercise without raising an error.
Use the method `obj_create` in `ExerciseLogResource` and `AttemptLogResource` classes at `main/api_resources.py` 

Refactored method signature of classes at `main/api_resources.py` to follow the method signature of its base tastypie classes  like `Resource` and `ModelResource`.