Update set of random galaxies

[sauln]

From black_holes_frontend created by edbreeden : codeforgoodconf/black_holes_frontend#3

Once user models are created, have the algorithm that chooses which galaxy to show a user dependent on that user.

It should

• Not show the same galaxy more than once
• Show some galaxies that we have already labeled (as a control set for the user)

[sauln]

Requires issue #13