Refactor CSV/DB writing with logging in course_inventory

[ssciolla]

The code that writes data to a CSV or the DB in course_inventory/inventory.py and logs before and after is quite repetitive and can likely be made into function(s) or class(es) to reduce duplication and errors.

Note that this is pretty closely tied to whatever method we chose to use for writing to the DB (pandas, SQLAlchemy, etc.), so some consideration should be given to whether the current method is what we will use going forward.