Open world cross-validation

[redshiftzero]

We should implement open-world cross-validation - and the ability to define how much of the world the adversary can see. Both these config options (world: type: 'open' and world: observed_fraction) and corresponding fields to store them in the database (in models.undefended_frontpage_attacks) were created in PR #57. It would be nice if we could use existing code from one of the splitting methods here but writing custom CV may be necessary. If custom CV code is indeed necessary, it should definitely be unit tested.

Also: consider making the observed_fraction in the config take a list of possible values.