Updating the public identifier for models to leaderboard_public

[shehadak]

Currently, each model has an attribute public that specifies whether model scores will be displayed publicly through the BrainScore leaderboard. We want to add a new attribute specifying whether a submitted model's codebase is public. To avoid confusion between the two attributes, the current public attribute in the database, submission JSON files on Braintree, and website need to be updated to leaderboard_public.

The new attribute specifying whether a submitted model's codebase is public could therefore be called code_public.