Add Django Admin interface

Make sure the problems are visible in the Django admin interface so logged in users with staff status can see what is stored in the database.

(In the future, annotations should be visible here as well.)

No files, tests, or entry points are named. Start by locating the project's Django admin configuration and the stored problem model, then verify that staff users can view problems while non-staff users cannot. Treat annotation visibility as future scope.

Written by the indexing model from the issue text.