Refactor how model visibility works in index actions:
* Call `visible` in the controller instead of in model `search`
methods. This decouples model visibility from model searching.
* Explicitly pass CurrentUser when calling `visible`. This reduces
hidden dependencies on the current user inside models.
* Standardize on calling the method `visible`. In some places it was
called `permitted` instead.
* Add a `visible` base method to ApplicationModel.
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