Update utils.py

Previously the herein referenced AUTH_PROFILE_MODULE could not point to a model for a subpackaged app because this get_profile_model function splits on all periods. As we know get_model takes app_label and model as arguments so for a subpackaged app this would break as it will send 3+ arguments. This adjustment to use rsplit with a maxlimit instead of split correctly interprets the app_label and model by splitting on the last period in the string.