Respect model registry and experiment visibility setting

Problem

User reports: #381660 (comment 1815362035)

The issue I had was that in the project General Settings, under Visibility, project features, permissions, I had the toggle for both Model experiments and Model registry visible before enabling the model_registry feature flag. Toggling the Model registry (with and without the feature enabled) currently hard codes the accessibility to Only Project Members.

Solution

• We want "everyone with access" level for non-private projects to work, I think we should update "app/policies/project_policy.rb" accordingly for "read_model_registry"

• Allow guests on private groups and have better tests for private groups who can't have "everyone can access"