update naming conventions

`BenchmarkType` currently has a field [`visible`](https://github.com/brain-score/brain-score.web/blob/ea3314af143df5849f6ac93025d1c7297bc26e5e/benchmarks/models.py#L110) 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`.

Start in benchmarks/models.py at BenchmarkType.visible, then search the repository for references to that field and the leaderboard display logic. Rename the field and its uses to leaderboard_public, preserving the existing public-leaderboard behavior; code_public is explicitly deferred.

Written by the indexing model from the issue text.