This changes the `#stub_feature_flags` to be:

- `stub_feature_flags(ci_live_trace: false)`
  Disable `ci_live_trace` feature flag globally

- `stub_feature_flags(ci_live_trace: project)`
- `stub_feature_flags(ci_live_trace: [project1, project2])`
  Enable `ci_live_trace` feature flag only on the specified projects

Which does represent an actual behaviour of `Flipper`:

1. You can enable selectively: enable means create override
   for a specific gate
2. You cannot disable selectively: disable means remove override
   for a specific gate