Add widget with list of Analytics referrals

Site Kit should offer a widget that lists the top referrals for the site. This would also be beneficial on a per-page level, i.e. might be worth including in the single URL view.

Originally requested in #36

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## Acceptance criteria

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## Implementation Brief

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### Test Coverage

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### Visual Regression Changes

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## QA Brief

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## Changelog entry

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No files, tests, or entry points are named. Start by reading the original request in #36 and establish the intended scope for the site-wide and single-URL views; the issue is complete when it has explicit acceptance criteria for listing top referrals in both applicable contexts.

Written by the indexing model from the issue text.