Add a people trainer

[sauln]

From black_holes_frontend created by amjcosta : codeforgoodconf/black_holes_frontend#4

We could add a "people trainer" on to the front end. The app would display a picture that has been verified as a black hole or not a black hole, and once a user guesses the answer, the app tells them whether they got it right or not.

@simian201 feel free to add more information!

[sauln]

Requires issue #13 to be completed first.