Cleanups in base.Logic
* Use regular kwargs instead of **kwargs
* Fold getKeyNameFromFields into _keyName
* Fold getIfFields into getFromFieldsOr404
* Rename getKeyValues to getKeyValuesFromEntity
* Rename getKeyFieldsFromDict to getKeyFieldsFromFields
* Rename getFromFieldsOr404 to getFromKeyFieldsOr404
* Rename getKeyValues to getKeyValuesFromEntity
Patch by: Sverre Rabbelier
Small changes in Mentor model (we don't inherit from Reviewer anymore since model inheritance is not supported in datastore).
Patch by: Pawel Solyga
Reviewed by: to-be-reviewed