Uninstall if participant is underage

During the setup survey, uninstall if the participant is under 18.

Fixes issue #29 
Well... hm. That isn't really straightforward. The survey generator builds it up like this: 0 is because it's the first entry, then it adds a dash, then removes spaces and special characters from "17 years old or younger." So even if I made a constant, this would be the only place I'd use it.

I know this is brittle because it's relying on the survey generator. But I don't want to change how surveys are generated just for this one case.

Hmm.. did you have anything else in mind? Anyways I will keep thinking about it.