[feature] Enforce MAC address format for called_station_id and calling_station_id

[pandafy]

The MAC address format stored in called_station_id and calling_station_id can vary depending on the NAS. This causes inconsistency in values of these fields.

We can provide a setting which allows user to specify a format.

Using the clean method at model level, the code shall update the field values according to the user specified format.