Update merge request approval widget component to use the GraphQL subscription

Once #383653 (closed) is done and the GraphQL subscription is present, the approval widget component can subscribe to it so it gets real-time updates when the subscription is triggered. Ensure that it re-uses the same websocket connection that already exists on the MR page.

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Availability and Testing

Add feature spec to ensure approvals get added and removed in real-time without reloading page.