Save the values of the averaged (over k-folds) ROC curve in the database

[redshiftzero]

Right now we generate a ROC curve that is averaged over all k-folds, but we don't save the FPR and TPR in the database in models.undefended_frontpage_attacks. We should do that such that we can quickly determine what the FPR and TPR is at a given threshold. Note that these are currently generated and saved for each individual fold.