Fix broken Django admin: make `Prediction` model readonly

When a model has a ForeignKey, as Prediction does onto Minute, the Django
admin tries to list every possible value in a <select> dropdown. This doesn't
work so well with 500,000 possibilities.

At the same time as fixing that, make the Prediction completely readonly as
it's safer if it's only modified through the `import_predictions` command.

https://www.pivotaltracker.com/story/show/76505816 [Delivers #76505816]