Allow the Key Metrics and Audience Segmentation widgets to be hidden for view-only users

## Feature Description

At present, the Key Metrics widget can be hidden for an individual admin user by toggling a switch in Settings. However, view-only users don't have access to Settings, so cannot hide the widget.

The first iteration of Audience Segmentation will take the same approach.

We need to determine a mechanism that allows the widgets to be hidden for view-only users and apply it to both features.

One possibility could be to provide a cut down Settings area for view-only users which only has these switches (and perhaps some read-only visibility of other settings that the view-only user can see).

However, we could take another route for example changing the flow to allow an admin to override the view-only settings.

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## Acceptance criteria

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## Implementation Brief

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### Test Coverage

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## QA Brief

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## Changelog entry

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Start by examining the existing Key Metrics hide control and the planned Audience Segmentation visibility flow, then trace how view-only users reach widget settings. Compare a view-only settings area with admin overrides; done when both widgets can be hidden by view-only users and the behavior is covered by the relevant tests.

Written by the indexing model from the issue text.