update naming conventions

[mschrimpf]

BenchmarkType currently has a field visible which controls whether or not the benchmark is shown on the public leaderboard.

Since this naming is potentially confusing with visible or public/private code, let's rename this field to leaderboard_public. Later on, we can then use the naming code_public for the visibility of source code. This way, users can have e.g. benchmarks or models with code_public=False but still show them on the website leaderboard_public=True. Or vice versa, the code can be public code_public=True but the benchmark/model is not shown on the website for everyone leaderboard_public=False.