Model evaluation over a range of base rates

[redshiftzero]

We don't really know the base rate of SecureDrop usage: yay anonymity. However, this means that in order to see how many users an attacker would correctly and incorrectly flag we should evaluate each model's precision and recall over a range of base rates - and of course save the results in the database. Note that in PR #57 I created the field testing_class_balance to keep track of this in table models.undefended_frontpage_attacks.

[psivesely]

Note to get in contact with those Navy researchers who were trying to get some privacy-respecting data on onion service usage. While we won't know proportionally how many users visit SD, it would be useful to know how much traffic OSs get in general.