Do not track changes of AK interest counter

This fixes #97
Exclude interest_counter from AK history
Prevent saving of history record when increasing interest counter

Run the following action after applying the change/migration to remove existing duplicate history records (the ones without changes to tracked fields, hence caused by either saving without changing any field or especially the ones created due to changes of the now ignored interest_counter):
python manage.py clean_duplicate_history AKModel.ak